Refer to "mixed" precision in user-facing documentation

GROMACS has used mixed precision for many years, so we may as well be
accurate about it. In the code, we should continue to refer to single
precision for aspects of the implementation, but mdrun should not
be referred to as running in "single precision."

Also removed useless install-guide section from manual.

Change-Id: I3c26acedcdb391de50bdcb6a5e913ea0cfc32845