Smart rectifier

[mattkerlogue]

Currently there is only one rectifier, simple_rectify(), which conducts a simple pivot of the dataset into a rectangular frame, using the column number for headings.

The aim of a "smart" rectifier would be to work more easily with spreadsheets that aren't tidy, specifically:

• guess the row containing column headings, or use a row/headings provided by the user
• guess the data type of columns, or use types provided by the user
• skip specific rows

Users should still rely on unpivotr for complex data structures.