Use enum class for nbnxm locality

Use enum class for nbnxm locality

Converted the interaction and atom locality enum to enum class.
This exposed and fixed a few mixings of the two enums.
Move some files into the Nbnxm namespace.
Add local/non-local organization to the GPU timers.

Note that the use of the Nbnxm namespace outside the nbnxm module
is only temporary. This should all be replaced by methods.

Change-Id: I3c891b3b2b3e14d8175b63f4191f365a5cd64b18