Ion bugfixes and enhancements

[rkingsbury]

Summary

This PR contains some miscellaneous bugfixes and enhancements for the Ion class.

• Fixes Ion.from_formula has unexpected behavior when +/- precede charge number #2281
• Partially address Inconsistency between formula and reduced_formula for diatomic molecules and Ions #2204 by overloading get_reduced_formula_and_factor to provide special formula handling for ions. The reduced_formula for hydroxide, hydrogen peroxide, acetate, acetic acid, and small alcohols are now more consistent with the way these species are usually written.
• Implement a utils.string.charge_string method to make the representation of ion charges more consistent across formula, reduced_formula, etc.
• Make representation of Ion formulae more consistent. By default, charges are now always appended in brackets, with the sign preceding the magnitude, e.g. Na[+1] or SO4[-2]. This provides unambiguous differentiation between stoichiometric coefficients and charges, and will facilitate using Ion.reduced_formula as database keys in, e.g., the Pourbaix ion data. For uncharged Ion, the formula is always followed by (aq).
• Change display of ions that may be hydrated metal complexes, e.g. Zr(OH)4(aq) becomes ZrO2.2H2O(aq)
• Add extra tests for to_latex_string and Ion.from_formula
• Add type hinting

Additional dependencies introduced (if any)

None

Checklist

• Code is in the standard Python style. The easiest way to handle this
  is to run the following in the correct sequence on your local machine. Start with running
  black on your new code. This will automatically reformat
  your code to PEP8 conventions and removes most issues. Then run
  pycodestyle, followed by
  flake8.
• Docstrings have been added in the Google docstring format.
  Run pydocstyle on your code.
• Type annotations are highly encouraged. Run mypy to type check your code.
• Tests have been added for any new functionality or bug fixes.
• All linting and tests pass.

[coveralls]

Coverage decreased (-0.6%) to 83.115% when pulling d378d27 on rkingsbury:ion_fixes into acfe589 on materialsproject:master.

[mkhorton]

Thanks for this PR @rkingsbury ! looks great.

One concern is the proliferation of special cases for organics, I wonder if there's a better long-term solution to this that's more general/transferrable.