calculation of colourmaps for stack_rate and stack_error 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).
• deleted the file PyRate/utils/colourmap.txt which is not needed anymore.
• added functionality to merge.py to generate a quickview image and kml-file for stack_error.tif, A white-red colourmap is used to display the velocity error saved to colourmap_error.txt in the 'out' dir.