Add basic gro file reader

[mattwthompson]

http://manual.gromacs.org/current/reference-manual/file-formats.html#gro

• Read name of system
• Read name and position of each atom
• Read box information (with caveats forced by GROMACS itself)

Note that residue information and atom numbers are not stored anywhere (though I added dummy variables to read them in)

[mattwthompson]

I updated this just a little bit - in 622f19d I decided it's better to explicitly state the units, even though they'd be converted from float to u.nm without much trouble, if only to avoid a UserWarning. But it's probably best to do that anyway.

I would like #35 merged in before this; I think it would make checking the box simpler and make other comparisons that we may wish to do easier.