Revert "HostCUDA.cu: use sqrt(1.0/rsq) instead of the approximate rsqrt(rsq)"

This reverts commit 75fbd14788fd30406dd090805d922b3ab9777be8.

See <https://github.com/insaneinside/changa/commit/75fbd14788fd30406dd090805d922b3ab9777be8#commitcomment-5837323>
for the discussion.
Some header changes for Charm-less unit tests, missing #includes

Backported from cuda-scaledmoments branch.
calculateRadiusFarthestParticle: constness fixes

These changes allow use of `const'-qualified particle instances with
this function.

Conflicts:
	MultipoleMoments.h
split off CUDA version of `momEvalFmomrcm' into its own file

This allows for easier unit testing.
Makefile.in: fix build when CUDA is not enabled

Since we don't generate cuda.mk unless CUDA is enabled, we need
a conditional around the include statement in Makefile.in.
Allow disabling HEXADECAPOLE at configure time.

Although normally this is a bad idea, the purpose is to allow reverting
to the old CUDA code (with is incompatible with the hexadecapole code).