Cutoff function can move to utilities or somewhere

[Alireza Khorshidi]

Cutoff function is also defined inside each fingerprint script, which makes it necessary to have the same copies inside all descriptors.

It can be moved to somewhere to be generally used.

[andrew_peterson]

That's a good idea. I think we should have it in desciptor/__init__.py or descriptor/cutoffs.py. We can have multiple types, and this will also make it easy for the user to try out their own ideas, like when you did with polynomials. I'd envision behavior like the following:

descriptor = Gaussian(cutoff=Cosine(6.5))  # Behler form
descriptor = Gaussian(cutoff=Polynomial(4, 6.5))  # Khorshidi form
descriptor = Gaussian(cutoff=6.5)  # Uses the default

The default (last line) above could be either choice, and we can decide for each descriptor what makes sense as the default.

[Alireza Khorshidi]

Implemented in the commit e0f93ff.