Dppd

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fixed norm_0_to_1()

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- added .dir_dppd() to list just dppd registered verb, not those wrapping the object itself.
- dp(DataFrame).insert() now returns self and is therefore chainable.

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- added .debug to print head&tail inline

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- allow dp(collections.Counter).to_frame(key_name='x', count_name='y').pd

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- pca() on dataframe
- 'natsorted' for categoricals
- select_and_rename (select no longer renames!)
- level selection in column specifications
(by passing in a list of regexps, might be shorter than the number of levels)

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- added binarize
- added dp({}).to_frame()
- minor bugfixes and polishing
- improved docs a bit