Clarified GPU selection output and mdrun help

The reporting of GPU selection has been confusing when devices are
shared as GPU IDs would show up multiple times in a list of "devices
selected to be used." The reporting has been modified to print the
number of devices selected followed by the GPU to PP rank mapping which
is in fact exactly the previously printed list of IDs.

Additionally, the mdrun help page now explicitly states that the GPU ID
string passed with -gpu_id specifies a per-node GPU to PP rank mapping
and that multiple ranks can share GPUs.

Change-Id: Id98c592c1dd38573df003247281e4edf50debba7