Allow user tweaking of FKM nonlinear constants

[maierbn]

Is your feature request related to the way how to work with pyLife, i.e. the API.? If yes, please describe.
Is your feature request about new functionality, new calculation model? If yes please describe
For fkm nonlinear, the constants and material groups are fixed. I would like to change the constants or add new parameter sets for other material groups.

Describe the solution you'd like
The assessment_parameters should have an additional function hook that is called whenever constants are accessed.

Describe alternatives you've considered
The current only alternative is to download a copy of pylife and change the constants yourself.

Indicate if you or someone you know would volunteer to work on this
I'm going to propose code how to do this.