Check for conflicting binarisation

[johenglisch]

Work on feature checks:

• Add unit tests for all the existing feature checks.
• Fix a few checks where I checked for the integer 0 instead of the string ‘0’.
• Check if binarised features match up.
• Complain when binarised ternary features are both 0.

[HedvigS]

are we waiting for @xrotwang to review too?

[xrotwang]

are we waiting for @xrotwang to review too?

I think I'm no longer on top of things in detail when it comes to pygrambank.