Support non GPU-aware MPI

[luraess]

The only change needed would be to replace

Chmy.jl/src/Distributed/exchange_halo.jl

Line 27 in c011eef

init!(tle, Architectures.get_backend(arch), dim, side, fields)

by

init!(tle, CPU(), dim, side, fields)

In addition, one may want to pin (page-lock) the CPU buffers in this case.

Unsure about the mechanism to select GPU-aware or not, given that there is no implementation agnostic way to check whether this functionality is enabled.

EDIT: actually, one could use LocalPreferences mechanism for that.

[luraess]

Closing this as addressed by #85 .