refactor `getPrefix` and `functionalQA`

[Remi-Gau]

refactor getPrefix and functionalQA

In src/getPrefix.m, line 3
| function [prefix, motionRegressorPrefix] = getPrefix(step, opt, funcFWHM)
|                                            ^^^^^^^^^ metric: exceeded cyc: measured 17 > limit 12
In src/QA/functionalQA.m, line 3
| function functionalQA(opt)
|          ^^^^^^^^^^^^ metric: exceeded cnest: measured 5 > limit 4
MISS_HIT Metric Summary: 161 file(s) analysed, 2 metric deviations(s)
Error: Process completed with exit code 1.

Originally posted by @Remi-Gau in #237 (comment)

[Remi-Gau]

for getPRefix from #161

This function has become a headache to work with.

Basic philosophy has to redesigned: currently it takes as input the name of COMING step in the pipeline and look for the prefix of the images ouputed in the PREVIOUS.

Example:

step 1: slice timing
step 2 realign and unwarp
getPrefix('normalise') should return au

Problem is that with new options and as ways to combine the "previous steps" evolve, the number of cases to keep track of "explodes" and makes this function very unflexible.

So I think it will be more flexible to have a less ambitious but more flexible function that can return the prefix of the previous steps and where the order of the previous function is in the call of the function or in opt.

step 1: slice timing
step 2 realign and unwarp
getPrefix('unwarp-stc') should return au

or

step 1: slice timing
opt.orderBatches.stc = 1;
step 2 realign and unwarp
opt.orderBatches.unwarp = 1;
getPrefix(opt) should return au

This will break a lot of things as this function is called in a lot of batch creating functions: might need to write some more tests to detect which ones break.