fix(pandas): gather default selection is all columns

[machow]

This PR fixes gather to behave more like in dplyr:

• gather now will gather everything if no selections are specified (addresses #448)
• gather with grouped data attempts to re-apply any group keys remaining in the result:
  • group keys still in result data are kept
  • the measure or value var are not considered when re-applying a key
  • if no group keys can be re-applied, an ungrouped DataFrame is returned