ICA explained variance in original data

[rcassani]

This PR adds code to compute the variance explained by the IC when using ICA decomposition.

The reasonable place to store these values is the SingVal field in the projector structure. However, this changes for back compatibility are needed as this field is currently either the PCA explained variance, or a string to identify other projectors ICA or REF (montage).

brainstorm3/toolbox/process/functions/process_ssp2.m

Lines 947 to 952 in e532c6d

	% There are 5 categories of projectors:
	% - SSP_pca: CompMask=[Ncomp x 1], SingVal=[Ncomp x 1], Components=[Nchan x Ncomp]=U
	% - SSP_mean: CompMask=1, SingVal=[], Components=[Nchan x 1]=U
	% - ICA: CompMask=[Ncomp x 1], SingVal='ICA', Components=[Nchan x Ncomp]=W'
	% - REF: CompMask=[], SingVal='REF', Components=[Nchan x Ncomp]=Wmontage
	% - Other: CompMask=[], SingVal=[], Components=[Nchan x Nchan]=Projector=I-UUt

Tentative solution:

1. Use the Comment to identify the type of projector
2. Always require the projector Comment to have the type of projector on it

References:
mne-tools/mne-python#11141