Fixed GB interactions for release-4-6

Release-4-6 was not using the correct GB kernels,
but the non-GB electrostatics cutoff kernel. In addition,
the accelerated kernels unrolled more than a factor 2 had
a bug that caused a small error in the forces. Both these
issues have been fixed, and the GB results now match
release-4.5.
Note: The GB _radius_ calculations are still only done
in C, but the accelerated options will be re-enabled in
a pending patch (separated for clarity).
Fixes #1096.

Change-Id: I40151b9a9f1920006bfe9a39a8719698a824bfac

fix

Change-Id: I45630252d6a9c05a7f3695a7e6d07dcd13e0a25c
Don't cache ${FFTW}_HAVE_(SIMD|AVX)

The error message was incorrect if the library was changed. E.g.
if the library wasn't found on the first try and then (e.g. with
CMAKE_PREFIX_PATH) was found on the 2nd try. Could have been
resolved by using FORCE when setting variables, but that would
have defeated the purpose of allowing the user to overwrite
the value for future versions which might be misidentified.
Any solution which lets the user to overwrite but still works
correctly if the library changes would be difficult. Given that
it is only for displaying a warning message, it is not worth
making it much more complicated.

Change-Id: I0b6c5a204ee0114ef9de9337491a14457917f9d7
Made g_tune_pme work correctly again with thread-MPI mdrun

Up to now, g_tune_pme would put the -nt switch on the mdrun command
line to ask for thread-MPI processes. Due to changes in mdrun, this
now leads to the fact that all runs with separate PME nodes exit
because the number of MPI threads must be explicitly specified with
the -ntmpi switch. Now the g_tune_pme -nt option is renamed to -ntmpi.
The remaining changes are only minor code cleanup / formatting things
plus getting rid of unused variables.

Change-Id: I7518799ef593d78d1cce1496852f0241e6d01755