add function to check that y is binary in dataframe_checks

[solegalli]

Many transformers in feature engine require that y is binary. At the moment we do this check within each transformer. We should create a function that unifies this behaviour and call it from the transformers instead of repeating all the logic every time.

Example: encoders woe, pratio

[Morgan-Sell]

hola @solegalli,

This check will be very useful! I'll work it on. Hopefully, it will be ready for you to review once you return from vacay ;)

[Morgan-Sell]

Being fixed in #484

[solegalli]

Actually, PRatio will be deprecated, so the only transformer atm using binary classifcation exclusively is WoE.

Not worth the effort.