Move utilities.h to Core.

I'll need this in core's elements.cpp soon...

Change-Id: I57d2f85f746f9d22eb6759dee8beff27ce133969
Add support for custom elements.

These will be used to track atoms from, for example, MD runs
where user labels are used to identify particles. It basically
reserves the atomic number range [128, 254] for such cases.

Change-Id: Iff80f116e408c6357a694072550e993d4a8bd4e2
Add simple GROMACS .gro file reader.

Currently reads atom positions and cell dimensions. Atom types
are all custom at this point -- we can add some GUI bits to let
the user remap the custom types into something real.

Change-Id: I4f6f23408fd49ca91ee992a111eec2ec87e69d11
Merge topic 'gromacs' into master

54683fa1 Add simple GROMACS .gro file reader.
e9be9e22 Add support for custom elements.
dee2e65e Move utilities.h to Core.