Allow for a fit of multi-component J(y) (a sum of different models, e.g. Gaussian+ Multivariate)

[GuiMacielPereira]

Problem
Currently Matthew is running his own Python script after the analysis has run that fits a J(y) function that is a sum of two gaussians. Matthew wants to implement and option for a J(y) fit of a composite of different models e.g. Gaussian+Gaussian+Gram-Charlier, Gaussian+Multivariate, etc.

Solution
The first step to address is to ensure all the individual functions required are available to fit the J(y) profile. Then somehow allow for a composite function to be made (which I think might be easier to achieve with Mantid than iminuit)