Added support for four different multi-colour pattern algorithms:
doubleHeight - this is the original algorithm which requires the use of the pattern variation switch 4 to double the height in order to work with a colour changer.
offsetRows - this is the most optimised method which uses the second pass of a colour as an opportunity to begin the next row if the previous was completed. This is the default method.
blankSecondPass - this simply adds a blank pass after each colour to enable the carriage to return to the colour changer.
ditheredRows - this breaks up the colour allocation to every first instance in the first pass of the carriage and every second on the return pass.
Full details and a benchmark between these algorithms can be found on this blog post: http://heartofpluto.co/2017/04/02/algorithms-of-multi-coloured-knitting/