MFX is not working with multisession

[lionel-rigoux]

MFX cannot be used to invert multisession data. As the multisession option is duplicating the priors, the posterior and prior do not have the same dimensions. This crashes MFX as it derives iteratively the prior from the posterior.
A potential solution would be to allow to specify priors for multisession data using the extended scheme (priors on all parameters, including session-wise duplicates).