Add rewriting logic and other operators (split, gather) to pandas API

[shreyashankar]

We only support map, filter, reduce, and equijoin in our Pandas integration. We should add the split and gather operators, as well as our rewriting logic.

Adding rewriting logic may be quite complicated, because user pipelines may have operations beyond the semantic operators. But we should at least be able to add the split and gather operators.

[Sansh2356]

Is this open for contribution @shreyashankar ?

[shreyashankar]

PR is out here: #308

Still have some tests to write

[Sansh2356]

@shreyashankar do we directly start working on any of the open issues or ask on which to work ? I am new so asking beforehand, it would give more clarity that's why.

[shreyashankar]

Good question -- i think it's good to ask on which to work; I just somehow missed this in my notifications. Sorry about that!