No support for derived type extensions

[domcharrier]

Issue:

Fortran derived types can be based on other types.
GPUFORT does not support this yet.

Background/Example:

type mytype1
   real :: a
end type

type, extend(mytype1) :: mytype2
   real :: b
end type

mytype2 inherited member field a from mytype1.

Assessment:

Issue has low priority as we did not see such code yet in any HPC
application that we tried to port.