Added gen and format information to AbstractBattle

[caymansimpson]

I did a couple of things here:

1. _format was an internal variable that wasn't used, and recorded the generation of the AbstractBattle, so I changed _format to hold the format of the battle, aligning with Showdown protocol via the python equivalent to Showdown's parsing
2. Stored generational info (the int) in _gen
3. Added gen and format as properties of AbstractBattle
4. Added a RunTime check to ensure that AbstractBattle is initiated with the right gen
5. Added to fixture_data official showdown logs for easier parse_message testing
6. Added testing for battle.format and battle.gen

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.05%. Comparing base (f458350) to head (aa0f974).
Report is 50 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #558      +/-   ##
==========================================
+ Coverage   83.38%   85.05%   +1.67%     
==========================================
  Files          39       42       +3     
  Lines        3918     4189     +271     
==========================================
+ Hits         3267     3563     +296     
+ Misses        651      626      -25     

[hsahovic]

Great change!