Don't require unit if integer is dimension spec

The problem

If you Fortran function/subroutine that we're wrapping takes an integer size parameter (i.e. an argument which says how big some array should be), we currently can't wrap it with fgen. The reason is that fgen expects every integer to have a unit, but obviously size parameters don't need units.

Definition of "done"

• Decide if we want to support passing in these size parameters when wrapping things with fgen (maybe this is a pattern we simply don't want to support)
• If we do want to support this, update the internal logic so that integers don't need units (perhaps if some indicator flag is set?)

Additional context

Came up in !25 (closed), but only because I was hacking around something else I hadn't got right (hence why this isn't obviously a problem we need to fix, at least not to me).