Improve and expand the functionality of multi-state global fitting

[HKaras]

There has been a few-recent publications using DEER to investigate how populations of different conformations change under different conditions, such as https://www.nature.com/articles/s41467-026-70027-9.

DeerLab is capable of fitting these multi-state models, but it is currently slightly-challenging and hidden as advance option, example: ex_global_twostates_parametric.py. This should be simplified by adding a function that can generate the necessary DeerLab models. The user should not need to handle the somewhat complicated model creation.

• Add a model creation function for the Pmodel
• Generalise the example for N-states
• Add uncertainites to the plots in the example