Adding enumerations for the target property of moves

[caymansimpson]

Adding enumerations for the .target() field in Move class, to make it easier to embed moves via OHE.

Changes:

1. Created an enumeration class of Target
2. Changed move.target to return this enumeration
3. Added unit tests to test the enumeration
4. Added unit tests to test target in moves

All unit tests passed in both test_move and test_enumeration

This is in response to issue #525, but only addresses the Target field because you already have encoded volatile statuses in effect.py

[caymansimpson]

Please reject if you think this is not well-thought through. I'm not sure how this should best jive with Issue #523, and specifically the DoubleBattle implementation here and here

[codecov]

Codecov Report

Merging #529 (2d4cf46) into master (f458350) will decrease coverage by 0.14%.
Report is 27 commits behind head on master.
The diff coverage is 89.09%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #529      +/-   ##
==========================================
- Coverage   83.38%   83.24%   -0.14%     
==========================================
  Files          39       40       +1     
  Lines        3918     3988      +70     
==========================================
+ Hits         3267     3320      +53     
- Misses        651      668      +17     

[hsahovic]

@caymansimpson thanks!