Add Fortran interface

[awehrfritz]

Description

This add a Fortran interface for OCCA and respective tests and examples.

The implementation utilises the Fortran 2003 ISO_C_BINDING module. Thus a Fortran 2003 compliant compiler is required. The interface relies on OCCA's C interface and is sthus limited to the functionality exposed via the C interface. The Fortran implementation is in most cases a simple interface to the C functions, thus simply providing a standardised way for the compiler to call the C functions from a Fortran program.

The Fortran interface is compiled into a shared library libocca_fortran.so, which can be simply linked to an application intending to use OCCA. Alternatively, the Fortran modules can be directly included in a statically compiled program, as demonstrated in examples/fortran/03_static_compilation.

This resolves issue #104.

Future work

While this initial implementation is fully functional, a subsequent revision could include additional wrapper/convenience functions to make the interface more "Fortran-like".
The following lists a few examples in this regard:

1. The Fortran-to-C string conversion could be hidden by implementing a small shim layer around the interface in order to hide the F_C_str function in:

device = occaCreateDeviceFromString(F_C_str(info))

2. Another example would be the Fortran pointer assignment for UVA memory, which could be wrapped in a single function call. Currently this requires the following:

  type(C_void_ptr)       :: a 
  real(C_float), pointer :: a_ptr(:)

  a  = occaTypedUMalloc(entries, occaDtypeFloat, C_NULL_ptr, occaDefault)

  if (C_associated(a)) then
    call C_F_pointer(a,a_ptr,[entries])
  end if

3. The call to C_loc in

call occaCopyPtrToMem(o_b, C_loc(b), occaAllBytes   , 0_occaUDim_t, occaDefault)

could be hidden in a convenience function/wrapper analog to the proposed shim layer in 1.

[dmed256]

🔥 🔥 🔥 This is pretty amazing, I'll take some time this week to check it out. Thank you!!

[dmed256]

Sorry it took a while. I have a few changes I would like to make but nothing blocking.

Thanks again for all your hard work!