Support allocatable non-array Fortran Types

[hiker]

ATM, allocatable is only supported for arrays (as it depends on having DEFERRED as array shape). In #2889 I have started to add an is_allocatable attribute to ArrayType and UnsupportedFortranType (the latter because we need this in lfric_atm) only.

While afaik there is no immediate need to support this, I'll add references to this issue so we can remove some if-tests if we ever support this.