Add to_array method for exporting boxes to GSD

[janbridley]

Description

Freud boxes are typically initialized from Lx, Ly, Lz, xy, xz, yz data, but there is no neat way to recover that data from the input. This method (which could probably should be renamed for clarity) provides that functionality.

Motivation and Context

Initializing GSD boxes can now be achieved in one function call:

frame = gsd.hoomd.Frame()
- frame.configuration.box = [freud_box.Lx, freud_box.Ly, ..., freud_box.yz]
+ frame.configuration.box = freud_box.to_array()

How Has This Been Tested?

New tests have been added

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the Freud Contributor Agreement.
• My name is on the list of contributors in the pull request source branch.
• I have updated the Change log.