Fix warnings from GCC 5

Fix warnings from GCC 5

Removed unnecessary ddata array from PME-spread spline
calculation. That might even be a useful optimization, for really bad
compilers. Moved declarations into the appropriate loops, so that
analyzers can work better here.

Added several other assertions to keep the code analyzers happy.

Reorganized implementation of get_vsite_masses to get cam declared and
used in a way that keeps analyzers happier. Removed a now useless
assert on j >= 1.

Change-Id: I9c5113f7e4013c629fca87614af4b74e249ee27a