Merge simple and condition schedulers

[kmantel]

The behavior of condition_scheduler with no conditions matches that in simple_scheduler. f060092 is optional but I think desired based on conversation with @jdcpni and @davidt0x

[pgleeson]

I think it makes sense to merge these, glad it was so straight forward.

One thing that would be nice though is if there was a wrapper class around the PNL specific classes/fields so that all the terms used (trial/timescale/sequence) are MDF's own and can be changed/updated easily on our side without impacting PNL. Also the "innards" of that wrapper class can eventually substituted with a MDF specific implentation (or a standalone schedluer) without too much additional changes...

[pgleeson]

Thinking a bit more about this I feel this file (modeci_mdf/scheduler.py) is the proper place for the "wrapper" class/fields (i.e. the MDFScheduler) which hides the specifics of PNI in an MDF specific API, and the EvaluableGraph could be extracted out & just use that API...