Allow for re-parameterisation of models

[Karen Edler]

Karen has requested the ability to easily re-parameterise models. i.e. generate a new parameter set for a model where there are new parameters and some are some function of the other parameters.

e.g.

new parameter: solvent_fraction
new parameter: component_sld
old parameter: solvent_sld
old parameter: sld = (1-solvent_fraction)*component_sld+solvent_fraction*solvent_sld

Thus the new parameter set would not have sld as a parameter, but have solvent_fraction and component_ld instead.

This should probably be implemented in a similar way to the 'New Custom Model' page.

[butler]

With the new model interface this may be less of an issue since you can do the same thing relatively easily by editing the model: change the python parameter list then at the top of the calculation code add the calculations needed to obtain the old parameters from the new parameters passed — the rest should then just work. Thus this may maybe should become moot? At least am going t lower to lowest possible priority in the meantime.

Note there is a related ticket regarding constraints #895