The kernel used only one grid dimension to loop over the periodic
points. This, however, fails if more than about 65000 points are in
the periodic boundary. Rearrange the grid to split this number over the
second and third dimension to avoid this issue.

Replace all instances of int(x, i4), int(x, i8), u/lbound(x, i4), u/lbound(x, i8), real(x, r4), real(x, r8) and cmplx(x, r8)

Replace all instances of _i4, _i8, r4 and _r8. Note, this affects routine names as well as precision suffixes.

Update OPTION variables to also use standard fortran precision, _int64

Replace declarations using kinds aliases with fortran standard variables. Note, formatting has not been preserved, but it's only cosmetic.

Make some pointers contiguous.

Execute the kernel only if there are boundary points. This avoids the
segfault that occurs when trying to access a size-0 array on the GPU.

fix indentations
remove trailing white spaces

Tune some OpenMP loops to improve memory access (by reducing NUMA) and using OpenMP simd more often.

The phase correction is now a buffer stored on the GPU during the full calculation. This reduces significantly the number of transfers for periodic systems.

Avoid potential integer overflows when computing the product of
pack_size along its dimensions or with other, potentially large numbers.
Add some more low-level routines to allow calling with i4 and i8
integers.

For large batch sizes, the multiplication of the number of grid points
with the number of states in a batch could overflow. Change the
integers involved to 8-byte.

This allows to encapsulate the communicator and also to use the MPI
wrappers defined for mpi_grp_t.

Especially when compiling with OpenMPI using the MPI module, the calls
should adhere to the standard, which is now the case.

Rename functions related to the par_vec_oct_m module to have consistent prefixes. Renamed pv_t to par_vec_t and its instances from vp to pv.

Add space between some keywords (if, case, then) and parentheses, so that the code resembles written human language more.