Shared base classes for demography classes

[davidorme]

Description

This PR:

• Introduces the demography.core module and defines the PandasExporter and Cohorts abstract base classes.
• The ABCs share a common class attribute ('array_attrs') which is used to define which fields to export to pandas.DataFrame and which fields need to be kept synchronized as cohorts are added or removed.
• Applies the PandasExporter to many of the demography classes and then updates the documentation to use the new to_pandas() method inherited from the base class.
• Adds some low level tests of some basic subclasses in tests.unit.demographt.test_core.py to show that the ABCs work independently and with multiple inheritance.

TODO:

• The Cohorts base class isn't used yet - need to think about whether to do this here. The main use case is within Community for adding new 'rows' to StemTraits, the cohort data etc. I think it's probably better to handle Revise cohort structure inside Community #336 first and then roll that ABC usage out.
• The Cohorts ABC name clashes with pyrealm.demography.community.Cohorts and needs renaming: CohortMethods or similar.
• Also need to use this in any partition of pyrealm.demography.community.Canopy attributes into cohort and community dataclasses (see Group internal attributes in Canopy into cohort and community objects. #337).

Fixes #314

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