Improvements to the API
* eps_sort can now handle all these kinds of input:
- DataFrame
- [DataFrame]
- ndarray
- [ndarray]
- generator
- [generator]
- generator of generators
- [double-subscriptable list of floats]
- double-subscriptable list of floats
- [double-subscriptable list of strings]
- double-subscriptable list of strings
* Added flag_nondominated function to return a list of booleans, or a
list of lists if it is called with a list of tables instead of a single
table