Dppd

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- Fleshed out reset_columns

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- added rename_columns/reset_columns

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- support for itertuples on groupBy objects
- column specs now support types (and forward the query to select_dtypes)
- column spec now accepts [True] as 'same columns, but sorted alphabetically'
- ends() for DataFrames
- categorize now by default keeps order as seen in the Series. Pass None to restore old
behaviour

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- a column spec of None now means 'all columns' (useful for e.g. distinct)
- added categorize verb for DataFrames

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- replaced alias_verb by extending register_verb(name=...) to register_verb(names=[...], ...)
- support for pandas 0.22.0

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- X is now 'stacked' - dp(...).pd now replaces the X with the one just before the last dp(..) call.