Clarify/remove the supported models from thesdk module

[mkosunen]

There are multiple reference to supported simulators in thesdk module. I think those should be removed. Instead we should define a package like 'rtl' as 'Provides support for models 'sv' and 'vhdl'

Suggestions:
If we want to use self.simpath, that should be overloaded in the simulator modules, OR not used in the simulator modules.

Motivation: Imagine that we have 10e6 supported simulators. Support for all of them should be defined in thesdk package for nothing.