Support pandas objects

[martinfleis]

Currently, we support pandas objects by a coincidence. We should make effort to support them officially without a need to pass the underlying array to silence warnings and ensuring that pandas support is tested and maintained.

[jGaboardi]

This should ideally be for all functionality, not only the gini stuff I mentioned in #102 , correct?

[martinfleis]

Yes, across the package and eventually federation. Though I believe that most components do support it already.

[jGaboardi]

@pysal/devs Does anyone else have any input?

[sjsrey]

One thought is we may want to consider a federation-wide policy on this.

I'm old enough to remember that we started before there was a thing called pandas (or numpy for that matter).

If we can centralize handling inputs, then it would make any future migrations less painful than what we have experienced in the past.