MT-CSD fractions rescaling with S0 response when normalizing

[rutgerfick]

MT-CSD doens't necessarily enforce unity on the volume fractions when fitting the signal. Since no penalty on non-unity fractions is imposed, this is the same as fitting the normalized signal E, enforcing unity on the volume fractions and fitting only the shape of the tissue responses, and then rescaling the volume fractions to correct for the differences in S0 tissue responses.

[rutgerfick]

Should be option of fitted SH framework, not a default action when fitting.