expand the encoder tests and double check them #11
Description
Describe the bug
I have a bit of fear that the encoders might be buggy and test are not complex enough. The encoders are very important and should be very clear to use.
We had big changes in Table constructor(pandas df index is not saved anymore), which could lead to buggy names in the one_hot encoder, because new names could now not be translated well and should be double checked.