Convert nbnxn_atomdata_t to C++

Convert nbnxn_atomdata_t to C++

Changed all manually managed pointer to std::vector.
Split of a Params and a SimdMasks struct.
Changed some data members to be private, more to be done.

This change is ony refactoring, no functional changes.

Note: minor, negligible performance impact of the nbnxn gridding
due to (unnecessary) initialization of std::vector during resize().

Change-Id: I9c70a1f8f272c80a7cf335fcbd867bd79c4102a2