MatrixType.realEntryType: save IntegerType instance
rework MatrixType isExact() / isGeneric() code
add common super type of all syntax elements
add new method to Interpreter interface
Checks: use common super type instead of EObject
slightly refactor checkParamTypes()
fix #33: interpretation of funcs with generic parameters

The issue was that types are not stored as strings,
i.e. names, but as MathematicalType instances. So
the interpreter did not recognize them as "generic",
as it only checks for a string with a leading '$'.