feat: add functional property for single-valued Data- and ObjectProperties

[markdoerr]

Dear linkml-owl developers,

for some tools, like the very popular owlready2, the import of properties requires a statement, if they are Functional Properties.
Could this be added to the linkml-owl output, if the Data- or ObjectProperty is single valued ?
(in case this might lead to side effects, which I do not overlook, a command line option --assign-functional-properties would be helpful).
In case you also consider this useful, I could make a proposal and submit a Pull Request.

Thanks :)