Change GMX_DOUBLE to be always defined

Change GMX_DOUBLE to be always defined

Also documented use of precision-related CMake variables

There may be some references in the SIMD documentation that still
refer to GMX_DOUBLE being defined, but we should solve that
separately, given the other changes at the moment.

Some abuse that un-defined GMX_DOUBLE was hard-coding some use of
single precision in mdebin_bar.cpp, so made that explicit.

Removed GMX_CPT_BUILD_DP, which can now be replaced by GMX_DOUBLE.

Removed some #ifdef in QM code, because floating-point arguments to
printf are automatically promoted to double. scanf still needs
versioning, though.

Refs #1855

Change-Id: I59b6874739d528603a86e0b6ab9bcadd67fdd3fa