Adding convert_water_mm_to_moles and calculate_water_molar_volume

[davidorme]

Description

This PR:

• Adds convert_water_mm_to_moles as the reciprocal of convert_water_moles_to_mm
• Adds calculate_water_molar_volume which otherwise is duplicate within those two functions.
• Adds tests - simple benchmarks for all three functions against real world approximate values and then extends the existing tests to check that the round trip through both functions works as expected.

Fixes #399

Type of change

• 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

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.48%. Comparing base (56ccf57) to head (ba35d89).
Report is 43 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #400      +/-   ##
===========================================
+ Coverage    96.27%   96.48%   +0.20%     
===========================================
  Files           34       35       +1     
  Lines         2659     2730      +71     
===========================================
+ Hits          2560     2634      +74     
+ Misses          99       96       -3     

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[MarionBWeinzierl]

LGTM, just one question about tolerances in the tests

[MarionBWeinzierl]

Is there a reason you are using atol here and rtol elsewhere?

[davidorme]

No 😄