improvements in element / chemistry handling

[rkingsbury]

Summary

A collection of enhancments and fixes to pyEQL's handling and reporting of chemical information

Major changes:

• Solution: add get_el_amt_dict method, elements and chemical_system properties
• fix some problems with the H2O(aq) entry in the database
• fix problem in get_amount that made it difficult to retrieve the pmg_ion property

Checklist

• Google format doc strings added.
• Code linted with ruff. (For guidance in fixing rule violates, see rule list)
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.
• I have run the tests locally and they passed.

Tip: Install pre-commit hooks to auto-check types and linting before every commit:

pip install -U pre-commit
pre-commit install