add function to save which last pybamm commit

[valentinsulzer]

Description

Function saves information about the commit, and if that is not available it returns the version number

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 #2293 (8026dac) into develop (2c60416) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #2293   +/-   ##
========================================
  Coverage    99.40%   99.40%           
========================================
  Files          364      364           
  Lines        20021    20037   +16     
========================================
+ Hits         19901    19917   +16     
  Misses         120      120           

Impacted Files	Coverage Δ
pybamm/__init__.py	100.00% <ø> (ø)
pybamm/batch_study.py	100.00% <ø> (ø)
pybamm/spatial_methods/finite_volume.py	98.15% <ø> (ø)
pybamm/spatial_methods/spatial_method.py	96.36% <ø> (ø)
pybamm/expression_tree/functions.py	100.00% <100.00%> (ø)
pybamm/spatial_methods/scikit_finite_element.py	98.85% <100.00%> (ø)
pybamm/util.py	100.00% <100.00%> (ø)
pybamm/models/submodels/interface/sei/base_sei.py	100.00% <0.00%> (ø)
...ybamm/models/submodels/interface/base_interface.py	100.00% <0.00%> (ø)
...m/models/full_battery_models/base_battery_model.py	99.78% <0.00%> (+<0.01%)	⬆️

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[priyanshuone6]

Looks good, thanks