Logical model should support nested structured terms with variables

[mkroetzsch]

The logical model currently supports arbitrary data values, which include nested structured ground terms (based on tuples and maps). This is also parsed, and actively used in @import and @export parameters.

The logical model needs to be extended to support similar terms even if not ground. Such terms can contain variables, and are therefore not captured in the datavalue model of nemo-physical. A possible economic solution would be to make the latter generic, so that it can also be instantiated with types that allow variables and ground datavalues to be used as fillers.

This is also a prerequisite for #464 and other issues that require higher-arity built-in functions.