Add BaseParameters and SymPy

[priyanshuone6]

Description

• Add Base Parameters class and tests
• Set Base Parameters as the parent class of all parameters
• Add SymPy in dependencies

(GSoC21: Print Equations)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Merging #1495 (e82ca77) into develop (fe6960c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1495   +/-   ##
========================================
  Coverage    98.34%   98.34%           
========================================
  Files          295      296    +1     
  Lines        17093    17104   +11     
========================================
+ Hits         16810    16821   +11     
  Misses         283      283           

Impacted Files	Coverage Δ
pybamm/parameters/base_parameters.py	100.00% <100.00%> (ø)
pybamm/parameters/electrical_parameters.py	100.00% <100.00%> (ø)
pybamm/parameters/geometric_parameters.py	100.00% <100.00%> (ø)
pybamm/parameters/lead_acid_parameters.py	100.00% <100.00%> (ø)
pybamm/parameters/lithium_ion_parameters.py	100.00% <100.00%> (ø)
pybamm/parameters/parameter_values.py	99.43% <100.00%> (ø)
pybamm/parameters/thermal_parameters.py	100.00% <100.00%> (ø)

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[valentinsulzer]

BaseParameters class looks good but test needs changing. In general it is best for unit tests to call the least number of methods possible (i.e. not require on other things not changing).
Also can you add the "GSOC21: print equations" label on the rhs? Or do you not have permissions to do that?

[priyanshuone6]

@tinosulzer I do not have the permission to add labels

[valentinsulzer]

Thanks @priyanshuone6 , I'll add the label for now and look into how to get you access

[valentinsulzer]

Also please update the CHANGELOG.md with your changes

[priyanshuone6]

Thanks @tinosulzer