Added IntegratorTwoStepNVTMTK (Martyna-Tobias-Klein integrator)

this is now the default for integrate.nvt
(mtk=True)
Added NVT MTK unit tests
Merge branch 'maint' into nvt_mtk
Minor strong scaling optimization in NVT MTK

use mapped memory for reduced temperature value
Fix compile error in test_gridshift
Really fix compile error
Fix potential seg fault

Don't try and abort MPI if the exec conf does not even exist yet
Fix exec_conf cache regression

exec_conf should be cached between multiple inits in a single script. This
used to be the case, not sure how it got reset again.
Fix an error in test_group

The new position set commands wrap particles, so the points at the right
edge of the box got wrapped to the left making apparent failures in
the test_group test.
Fix compile error on ubuntu 12.10
Added autotuners to NVT MTK
Restricted ComputeThermo workaround to non-MTK NVT
Forgot to set autotuner options
Merge nvt_mtk: fixes #1727

* codeblue/nvt_mtk:
  Forgot to set autotuner options
  Restricted ComputeThermo workaround to non-MTK NVT
  Added autotuners to NVT MTK
  Minor strong scaling optimization in NVT MTK
  Added NVT MTK unit tests
  Added IntegratorTwoStepNVTMTK (Martyna-Tobias-Klein integrator)