Improved Calibrate function

Rotated axes
One issue with the digitise package is that if the page to be scanned is
rotated so that the  x and y axes on the scanned page don't align with
the x and y axes on the screen then the Calibrate function will give an
invalid answer.

This is shown in the blog

http://lukemiller.org/index.php/2011/06/digitizing-data-from-old-plots-using-digitize/

The following function Calibrate fixes this problem by rotating the
digitized data so that the x and y axes from the figure correspond with
the screen axes