fix: norm_0_to_1

There was a stupid bug in the way norm_0_to_1 was implemented.
It should now be correct.

v.0.26

• 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.

v0.25

Added .debug() for inspecting dataframes without disturbing the pipe

Counter.to_frame

allow dp(collections.Counter(...)).to_frame(key_name='hello', count_name='world').pd

v0.23

Release 0.23 brings

• 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)