Parametrize offset models

[CeliaBenquet]

Parametrize offset models into a single OffsetNModel class so that we don't have to recreate a specific model for each new receptive field wanted. Instead, it can just be added to the list of parameters in the parametrize decorator.

This handles both odd and even receptive field sizes.

[stes]

Good initiative, thanks --- but add tests that confirm that the previously hardcoded models are 1:1 the same. You can do this by:

• copy pasting the old model code to the test folder as reference implementation
• instantiate with the new code, and check that all kinds of properties match 1:1

If this is hard to achieve, the alternative is to just let old and new models co-exist :)

e.g. use "varoffset-10-model" or so for the new ones?