Create core module

[MarionBWeinzierl]

This PR closes #75 . It creates the described core module and moves the utilities and hygro modules as submodules into the core. It also adapts the documentation to reflect these changes.

Updated by @davidorme:

I think we can now move this out of draft status and review it:

From #75:

• The core module is now created and includes utilities, hygro, water and pressure.
• Those last two submodules are the general functions that were in pmodel.functions.
• There is now a CoreConst constants class to cover core module functions. This also closes #76.
• The pmodel module method and function signatures are updated to add core_const where required, and then their code is updated to use the correct constant instances.
• There is a (minimal) api entry in the docs to expose the docstrings on the core functionality.

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (b9a454f) 93.38% compared to head (31fd2d3) 93.43%.

Files	Patch %	Lines
pyrealm/core/water.py	96.33%	4 Missing :warning:
pyrealm/constants/core_const.py	95.23%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #139 +/- ## =========================================== + Coverage 93.38% 93.43% +0.05% =========================================== Files 21 23 +2 Lines 1134 1143 +9 =========================================== + Hits 1059 1068 +9 Misses 75 75 ```

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[davidorme]

Looks good so far!

[davidorme]

I left some tests in pmodel that should properly now be in tests\unit\core. I had forgotten that #151 has already been merged into develop, so was leaving them until they could be moved into the new structure, but they can so should be here.