Determine whether to split into core and extension package

[keller-mark]

As we are doing in the R package for bioconductor submission, it may be easier to maintain a core package separately from an extension package which deals with the specifics of different data structures (i.e. the wrappers.py file https://github.com/vitessce/vitessce-python/blob/master/vitessce/wrappers.py)

See https://github.com/vitessce/vitessceR/issues/56 and https://github.com/vitessce/vitessceAnalysisR