Merge pull request #1 from drvinceknight/16332

Added a short paragraph at the beginning
Updated readme to indicate that this is a development repo
Fixed readme
Merge pull request #2 from drvinceknight/16332

Updated readme to reflect that this is a development repository.
Added payoff vector as attribute
fixes #5 by moving some documentation
fixes #6 and all tests pass, only 6 so far...
improves get_predecessors function
improves documentation
moves documentation and changes tests slightly
adds payoff_vector to test
creates show function
adds test to show function
creates is_monotone function
creates is_superadditive function
creates tests for is_superadditive and passes them
removes player_list from required attributes
sets up functions for properties of payoff_vector
change starting paragraph
creates is_nullplayer function