list all GSL root solvers that are used (do not hard-code them)
add documentation for initial guesser classes
more documentation for the model class
call CreateUnitMatrix[] only when needed

Reason: CreateUnitMatrix[] might return an error for some types.
Merge branch 'bugfix-unit-matrix-on-demand' into extend-ne6ssm-files
Merge branch 'extend-ne6ssm-files' of https://github.com/pathron/FlexibleSUSY into extend-ne6ssm-files
adding generated code temporarily.