Moved the Source emissions from dicts into lists

- The 'activity_emis', 'subgrp_emis', and 'subst_emis' attributes of
  the Source class is now deprecated and will produce a warning if
  used.
- The new list attributes that contains the emissions are
  'activity_emissions', 'subgrp_emissions', and 'subst_emissions',
  corresponding, respectively, to the former dictionaries.
- A new instance method of the Source class, 'lookUpSubst', can be
  used to look up a substance emission given a substance index.
- Another new instance method, 'iterSubst', can be used to iterate
  over all substance emissions with a given index.
Added a formula parser module, pyAirviro.other.formula
Added array support to the formula parser in pyAirviro.other.formula