create colourmap for quickview png image on-the-fly

[tfuhrmann]

• added functionality to merge.py enabling the calculation of a red-white-blue colourmap on-the-fly.
• the colourmap.txt file is created in the 'out' dir using the min/max range of velocity values present in the stack_rate.tif file, centered around 0.
• RGB values are automatically allocated to the velocity values using a colourmap containing 255 items ranging from red via white to blue (white colour == 0 velocity).
• the file colourmap.txt currently saved in the PyRate/utils folder is not needed anymore and can be deleted if you're happy with this PR.

[mcgarth]

Closing. Superseded by https://github.com/GeoscienceAustralia/PyRate/pull/273