Use DeviceBuffer in GPU update and NBNXM code

Use DeviceBuffer in GPU update and NBNXM code

... instead of raw device pointers and DeviceBuffer<float3>.

We try to use DeviceBuffer<Float3>, but in some places we have to use
DeviceVector<gmx::RVec>, until we can define FloatN types without
including any backend-specific headers. Currently, Float3 is defined as
gmx::RVec, so this should not cause any issues.

Also added some helper functions to convert RVec ̌<-> Float3 <-> float3.

Preparation for #3932 and #3941.