remove freq option in matrix

[majidaldo]

I'm seeing all the issues related to this option. I'm having issues with KeyError('Could not divide time index into desired frequency.'). The solution is to just remove this option. The index is the user's responsibility and the library should take it as is.

There should be a separate data structing step which would be followed by the viz.

[ResidentMario]

I think this feature is useful enough to warrant inclusion, but I concede that it's not the best thought-out library feature. Such are the pitfalls of accepting pull requests from strangers.

Rather than removing it outright, I plan to audit it and see what I can do the next time I work on this codebase. I suspect that I can get this feature to a good state with more careful error handling and strftime formatting for the labels.