Single precision support for MPI messages

[semi-h]

There is the compile time seleciton mechanism for the precision in the code base, however the MPI modules are currently hardcoded for MPI_DOUBLE_PRECISION. We need to figure out a way to fix this so that we can run single precision simulations.

A very incomplete attemt is here (albeit the wrong place, it should be in base backend)

x3d2/src/cuda/backend.f90

Line 26 in 85246f3

integer :: MPI_FP_PREC = dp

Then based on the dp parameter MPI_FP_PREC can be set and used everywhere afterwards. This probably will require converting mpi_sendrecv modules into classes, so that they can be instantiated at the backends with the MPI_FP_PREC

Thanks @JamieJQuinn for pointing out this in #67.

[pbartholomew08]

This is not the correct way to do it - dp = kind(0.0d0) (or something similar) will be compiler-specific, and not necessarily aligned with the MPI library. Something equivalent to

#ifdef DOUBLE_PREC
integer, parameter :: MPI_FP_PREC = MPI_DOUBLE_PRECISION
#else
integer, parameter :: MPI_FP_PREC = MPI_REAL
#endif

is what should be used (could also do with functions, but then can't make it constant)

[pbartholomew08]

it might also be worth looking into MPI_F08 - this is a better module than the default MPI one, however it isn't very well supported and tends to break tools so I haven't really used it... But it is a bit better on the type safety side of things.

[semi-h]

Parameter is actually very simple way of implementing this, and probably most robust. I think we can go ahead with it.