Easier re-use of nexus loaded material

[domna]

Current status: If a nexus dispersion file is loaded it is directly converted to a material, but the internal parameters are not directly accessible to be re-used in a fitting routine.

Possible solution: Expose the parameters and allow to update them. This may be a good general approach, to build a dispersion once and then only change its parameters during fitting rather than re-building the whole model all the time.