Add GRO writer

[mattwthompson]

Will require at least #40 and #49

[justinGilmer]

Small changes and comments

[ahy3nz]

Given that the gro writer will be an ongoing work in progress for a long time, do we want to throw warnings about the limited res_id, res_name, box functionality?

[mattwthompson]

At a very basic level, writing out box vectors for non-90/90/90 systems appears to be working (just looking at the output file from the unit test)

[ahy3nz]

Looks like a good milestone for a gro writer

[mattwthompson]

@justinGilmer just want to you look over how I'm handling the vectors, with the understanding it will be properly fixed later. Feel free to merge if it looks good to you.

[justinGilmer]

Looking great! Just a few nits and questions.

[justinGilmer]

added unit vector and scaled vector support with better names under #58.

[mattwthompson]

All tests passing for me, and the result is the same as above. Can you pull this locally, run tests, and if everything's good, merge? @justinGilmer

[justinGilmer]

will do!