Modify single/double EPS to agree with ISO C

The previous Gromacs values were a factor 2 too tight. While
we're at it, we also added float modifiers to the single-precision
constants, increased the accuracy in the double max/min values,
and improved the least significant bit in one of the single-precision
values. The previous relative accuracies for the PME correction
analytical approximations where set fairly arbitrary by dividing a
tightly chose constant with GMX_REAL_EPS, and since this is now a
larger number the unit test tolerance for those corrections has been
increased slightly to avoid triggering false failures.

Fixes #1356.

Change-Id: If29d2ff4141d86fd019e0dbdbfb7737ead28c3d4