Fix parallel optimisation and increase coverage

[BradyPlanden]

Description

Bugfix optimisation.parallel, add coverage tests, initial multiprocessing import and context set.

Issue reference

Fixes #298

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.79%. Comparing base (0273723) to head (fed4a12).
Report is 8 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #299      +/-   ##
===========================================
+ Coverage    95.49%   95.79%   +0.30%     
===========================================
  Files           38       38              
  Lines         2040     2045       +5     
===========================================
+ Hits          1948     1959      +11     
+ Misses          92       86       -6     

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[BradyPlanden]

This PR is now ready for review. There is a larger term discussion on how we handle Windows systems, as the multiprocessing doesn't work without wrapping the call structure within a __main__. If we can sort a method out for this,it would greatly increase our optimisation performance across all operating systems. A few results from local timing (Macbook Pro M3) of the spm_PSO.py example with iterations = 200.

	Time
Current	~9.9s
PyBaMM "fast with events" Solver	~9.0s
optim.set_parallel(True)	~3.3s

At the moment, this adds multiprocessing for Linux, MacOS, and Windows with WSL.

[agriyakhetarpal]

This looks interesting, @BradyPlanden, nice! I wonder if something like Dask can be used to fix what has been happening on Windows here (and also, given the recent parallel discussion on PyBaMM). It could be too much work to add, though 😬

[agriyakhetarpal]

Actually, this won't entirely work and might break further sometime. For example, if one does

import pybop
import sys

and then something like this

print([k for k in sys.modules.keys() if "pybop" in k])

gives us this:

tap to view output

['pybop._version',
 'pybop._utils',
 'pybop._problem',
 'pybop.costs',
 'pybop.costs.base_cost',
 'pybop.observers',
 'pybop.models',
 'pybop.models.base_model',
 'pybop.parameters',
 'pybop.parameters.parameter',
 'pybop.observers.observer',
 'pybop.costs.fitting_costs',
 'pybop.costs.design_costs',
 'pybop.costs._likelihoods',
 'pybop._dataset',
 'pybop.models.lithium_ion.echem_base',
 'pybop.models.lithium_ion.echem',
 'pybop.models.lithium_ion',
 'pybop.models.empirical.ecm_base',
 'pybop.models.empirical.ecm',
 'pybop.models.empirical',
 'pybop._experiment',
 'pybop._optimisation',
 'pybop.optimisers',
 'pybop.optimisers.base_optimiser',
 'pybop.optimisers.scipy_optimisers',
 'pybop.optimisers.pints_optimisers',
 'pybop.parameters.parameter_set',
 'pybop.parameters.priors',
 'pybop.observers.unscented_kalman',
 'pybop.plotting',
 'pybop.plotting.plotly_manager',
 'pybop.plotting.quick_plot',
 'pybop.plotting.plot2d',
 'pybop.plotting.plot_dataset',
 'pybop.plotting.plot_convergence',
 'pybop.plotting.plot_parameters',
 'pybop.plotting.plot_problem',
 'pybop']

so, running del sys.modules["pybop"] and running the above list comprehension just returns the above list, which is almost the same except the last entry, which gets removed:

tap to view output

['pybop._version',
 'pybop._utils',
 'pybop._problem',
 'pybop.costs',
 'pybop.costs.base_cost',
 'pybop.observers',
 'pybop.models',
 'pybop.models.base_model',
 'pybop.parameters',
 'pybop.parameters.parameter',
 'pybop.observers.observer',
 'pybop.costs.fitting_costs',
 'pybop.costs.design_costs',
 'pybop.costs._likelihoods',
 'pybop._dataset',
 'pybop.models.lithium_ion.echem_base',
 'pybop.models.lithium_ion.echem',
 'pybop.models.lithium_ion',
 'pybop.models.empirical.ecm_base',
 'pybop.models.empirical.ecm',
 'pybop.models.empirical',
 'pybop._experiment',
 'pybop._optimisation',
 'pybop.optimisers',
 'pybop.optimisers.base_optimiser',
 'pybop.optimisers.scipy_optimisers',
 'pybop.optimisers.pints_optimisers',
 'pybop.parameters.parameter_set',
 'pybop.parameters.priors',
 'pybop.observers.unscented_kalman',
 'pybop.plotting',
 'pybop.plotting.plotly_manager',
 'pybop.plotting.quick_plot',
 'pybop.plotting.plot2d',
 'pybop.plotting.plot_dataset',
 'pybop.plotting.plot_convergence',
 'pybop.plotting.plot_parameters',
 'pybop.plotting.plot_problem']

Therefore, we should delete all of these entries (i.e., pybop.*) before importing pybop again.

[BradyPlanden]

Thanks for the feedback @agriyakhetarpal. I've updated now, I believe this fully unloads the module. Let me know if you catch any other issues!