Allow for a list of variables to be read in as a dictionary instead of GC_var approach

[tsherwen]

Allow for a list of variables to be read in as a dictionary instead of the current (e.g. AC.GC_var()) approach.

Currently, variables are stored in an inline list/dict in variables.py, but this would be cleaner if stored in a yml file.

[tsherwen]

Done a test implementation for 3 functions in variables.py using a YAML read. Generally working fine.

Issues to fix:

• processing of LaTeX strings (e.g. read in of '$\mu$g m$^{-3}$' as '$\mu$g m$^{-3}$' (this will need to be fixed before using this method for latex_spec_name() calls )
• blocks of family assignments (e.g. subfamilies within spec_stoich() )