Refactor dataframe hierarchy with better names

[RehanSD]

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We want to refactor the Dataframe hierarchy with better names, to include the ModinDataframe.

[RehanSD]

This issue is motivated by this comment on the refactor pr #3717 that introduced the ModinDataframe and df algebra to the codebase.

The (paraphrased) resolution:

We could keep the name for ModinDataframe, since it just exposes the algebra, and then rename the PandasDataframe the PartitionedModinDataframe since it implements the algebra but also knows that its partitioned.

This will also require us to separate out the pandas logic from the PandasDataframe into a mixin.