Optional function-based API

[JasperVanDenBosch]

This is the idea to have an additional way to program with rsatoolbox where the user can pass data arrays directly into functions, as discussed previously alongside #160

from rsatoolbox.func import calc_rdm
rdm_vectors = calc_rdm(data)

We currently have no urgency in implementing this but this issue is here to keep the idea around.

[JasperVanDenBosch]

Today we discussed this as there is some renewed interest.

Two ways to implement this

1. inside-out: internal code should work on arrays/tensors, and the current object-based API wraps and unwraps these. The functional API then becomes trivial
2. outside-in: The functional API wraps the existing object-based API.

Two considerations for choosing between these

a) complexity, amount of work involved in refactoring
b) performance. Will 2) have a lot of overhead?

We should learn something about b) during our work on searchlight-style pipelines.