Enable fp-exceptions

Enable fp-exceptions

This can help with finding errors quicker because mdrun crashes as soon
as a floating point value overflows or is invalid. fp-exceptions are
only enabled for builds with asserts (without NDEBUG), mainly because
it isn't always possible to avoid invalid fp operations for SIMD math
without a performance penalty.

Also, fix a few places where we had 1/0 or other invalid fp operations.

Fixes #1582

Change-Id: Ib1b3afc525706f4b171564fcaf08ebf3b2be3122