fix internal/external OpenMP thread affinity clash

fix internal/external OpenMP thread affinity clash

Thread affinity set by the OpenMP library, either automatically or
requested by the user through environment variables, can conflict with
the mdrun internal affinity setting.
To avoid performance degradation, as Intel OpenMP has affinity setting
on by default, we will explicitly disable it unless the user manually
set OpenMP affinity through one of the KMP_AFFINITY or GOMP_CPU_AFFINITY
environment variables. If any of these variables is set, we honor the
externally set affinity and turn off the internal one.

Change-Id: I78c6347154d6f11695ee04243db17bbb2e5cb0a7