Move submodels and build model

[rtimms]

Description

Move general code about submodels to BaseModel instead of BaseBatteryModel. This makes it easier to build custom models from submodels.

Also fixes a bug where SPMe always build even when build=False.

Fixes #2056 #2168

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

[brosaplanella]

Looks good, thanks Rob!

[codecov]

Codecov Report

Merging #2169 (2f3d4d7) into develop (1cd386d) will decrease coverage by 0.02%.
The diff coverage is 95.00%.

@@             Coverage Diff             @@
##           develop    #2169      +/-   ##
===========================================
- Coverage    99.38%   99.35%   -0.03%     
===========================================
  Files          356      356              
  Lines        19309    19357      +48     
===========================================
+ Hits         19190    19233      +43     
- Misses         119      124       +5     

Impacted Files	Coverage Δ
...models/full_battery_models/lead_acid/basic_full.py	100.00% <ø> (ø)
...ybamm/models/full_battery_models/lead_acid/full.py	100.00% <ø> (ø)
...dels/full_battery_models/lead_acid/higher_order.py	98.16% <ø> (ø)
...ybamm/models/full_battery_models/lead_acid/loqs.py	100.00% <ø> (ø)
...bamm/models/full_battery_models/lithium_ion/dfn.py	100.00% <ø> (ø)
...bamm/models/full_battery_models/lithium_ion/mpm.py	100.00% <ø> (ø)
...s/full_battery_models/lithium_ion/newman_tobias.py	100.00% <ø> (ø)
...bamm/models/full_battery_models/lithium_ion/spm.py	100.00% <ø> (ø)
pybamm/models/base_model.py	98.53% <94.62%> (-0.74%)	⬇️
...m/models/full_battery_models/base_battery_model.py	99.75% <100.00%> (-0.02%)	⬇️
... and 2 more

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