Pathways interface simplification

[luisfabib]

See #39
Simplified the interface for defining dipolar pathways in dipolarkernel and dipolarbackground. Instead of using a matrix for defining the pathways, using NaN for unmodulated pathways, these are now defined as a list of pathways, each being a list of parameters.

Refactored and simplified code for both functions profiting the new interface.

[stestoll]

There seem to be some inconsistencies regarding pathinfo vs pathways across several files. I do like pathways, it's more directly linked to the underlying concept. I think we should use it throughout.

[luisfabib]

Yes, I like pathways better too. The current keyword is pathinfo which might be confusing.