Added MaskThreshold input

http://www.grasshopper3d.com/group/ladybug/forum/topics/honeybee-falsecolor
Initial commit

I added this new component based on a wish in this Grasshopper thread
(http://www.grasshopper3d.com/group/ladybug/forum/topics/honeybee-falsecolor).
The component let the user to convert different types of image files to
each other.