Another bugfix to 4e88b10c76bc30713c48762cc32b3d962f75d93a #936
Removed forgotten assert
Activated refactored IntegratorInternal::getDerivative, #936 #794
Revert "Revert "Cleaned up IntegratorInternal::getDerivative #936""

This reverts commit e422647f5f16094c10320073b7969c1ce3729282.

Conflicts:
	symbolic/fx/integrator_internal.cpp