IPython dependency -- remove it or make it optional

[jsstevenson]

Is your feature request related to a problem? Please describe.
The IPython dependency, and its sub-dependencies, makes the SeqRepo install footprint bigger than I'd expect it to be. On my mac, it adds about 25MB, although it's not the single biggest item (which is Pysam). I think IPython is included as part of the interactive console feature, which is neat, but probably not mission-critical or necessary for downstream apps/libraries to be installing.

Describe the solution you'd like
Either pull this feature, or make it an optional dependency group, so that downstream apps/libraries don't have to install IPython.