Add some type check in create_model

[RaulPPelaez]

When load_model or create_model are called explicitly there is no mechanism to enforce types of the arguments like it happens when train.py is used (via arg parse).

This means that something like:

model = load_model(..., cutoff_upper=5)

fails because cutoff_upper should be a float but it is passed down as an integer.

This PR casts to float when necessary to avoid this.

Closes #267

[RaulPPelaez]

@guillemsimeon review pls

[guillemsimeon]

awesome