Allow to Push and Pull Non Linear Load Cases

[GCRA101]

Description:

Currently the ETABS Adapter allows to push/pull only Linear Load Cases.
It would be important to allow it to push/pull also Non Linear Load Cases (including Response Spectrum and Time History Load Cases)

[GCRA101]

@peterjamesnugent , @Chrisshort92 , @IsakNaslundBh,
I think this is something that affects all Toolkits actually.
It would be great to have a catch-up to see how we want to tackle it.

[peterjamesnugent]

@GCRA101 it would just need an update to this Enum:
https://github.com/BHoM/BHoM/blob/develop/Structure_oM/Loads/Enums/LoadNature.cs

If you raise an issue on the oM, I can review it for you. We should raise an issue on all other toolkits once it's implemented so we can at least catch an exception (if it's not already dealt with a fallback method).