Add more visibility to decor. (#1953)

Functionality to
- delete ions from global properties
- peek into global properties
- expose painted, placed, and defaulted items
- expose voltage limit to python
- make voltage limit an optional to allow for arbitrary values.
- Add ion_settings view to python = (valence, ion_data, rev_pot_method)

Documentation
- cable cell properties in python

Closes #1578 
Closes #1577 
Closes #1607