Added current_observation property to AbstractBattle

[caymansimpson]

This is important for battle parsing when there's a force switch; an agent needs to be able to access the logs in the current turn to assess the battle state and understand what it needs to do next.

Mentioned here: #630

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.44%. Comparing base (f458350) to head (203e510).
Report is 80 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #639      +/-   ##
==========================================
+ Coverage   83.38%   85.44%   +2.06%     
==========================================
  Files          39       42       +3     
  Lines        3918     4302     +384     
==========================================
+ Hits         3267     3676     +409     
+ Misses        651      626      -25