File format support

[mattwthompson]

I don't think GitHub has the concept of a pinned issue, but I want to have a master tracker of the status of support for various file formats.

Extension	Engine	Typed or Un-typed?	Internal reader	Internal writer	External support
.xyz	many	Un-typed	✅ 8bc9360	✅ #46
.mol2	many	Either
.gro	GROMACS	Un-typed	✅#26	✅ #48
.top	GROMACS	Typed
.pdb	many	Un-typed
.data	LAMMPS	Typed		✅ #28
.gsd	HOOMD	Typed		✅ #47
?	Cassandra	Both?
.par	GOMC/NAMD? #78	Typed
.psf	GOMC/NAMD #78	Un-typed
.zmat	ilff/fftool	Un-typed

[mattwthompson]

Not sure if this should be in topology.py or elsewhere, but we eventually want to wrap each of these functions into a single, simpler one, like mb.load() or pmd.load_file(), that infers file type based on extension and calls the appropriate reader.

[mattwthompson]

We should think about how best to test our writers. Obviously we'll want to make sure the files get written, but it's not so easy to verify they get written accurately. Andrew brought this up in mbuild years ago (mosdef-hub/mbuild#217) and basically no progress has been made because it's hard to do. The only way I can think of breaking this up is

1. Make sure our readers can read the files back in
2. Make sure other readers can read the files back in (picking them on a case-by-case basis)
3. Sanity checks and agreement between the above
4. Evaluate the total potential of the system (much farther down the road)

[mattwthompson]

UMN's CP2K repo https://github.com/ramanishsingh/CSSI-CP2K