Write The Array Format

[william-dawson]

Currently, we support reading of both array and coordinate formats. However, we only support writing the coordinate format.

Supporting the array format would be a nice feature, though implementing it feels out of the scope of this project. The main challenge is that the data must be stored in a very specific order before writing. Whereas for the coordinate format, any order is fine, which keeps the data redistribution routines in the hands of the user.