Update TNG build system

GROMACS can now find and use a copy of TNG installed on the system.
The default is still to compile the version shipped with GROMACS.
That compilation is now handled using the BuildTNG.cmake script, so
there is no duplication between the GROMACS and TNG CMake code that
builds TNG, and the result is fully portable with all supported
environments and CMake versions.

Moved TNG handling into src/gromacs/CMakeLists.txt where it is
actually used.

Bumped the version of the TNG library to 1.6. Corresponds to commit
ba9aea42b01 in the TNG repository.

Renamed md5 things with GROMACS prefixes, so that there is no internal
name clash with the md5 things in TNG, and no client of libgromacs can
get unexpected behaviour.

Updated GMX_USE_TNG=off to work, and silenced unused-parameter
warnings associated with that setting.

Refs #1453 (partial fix)
Fixes #1520

Change-Id: I7ac9b578e45b8e75c67c9df9440eed968a3a9371