Repository name changed form mathics-omnibus to Mathics3-omnibus.

[rocky]

Over time, I'd like to regularize the Mathics3 repositories we have and start repositories with Mathics3 instead of Mathics, or mathics where it makes sense. For some repositories like stopit or the module extension to trepan3k, this does not make sense.

To that end, I just renamed "mathics-omnibus" to "Mathics3-omnibus".