Fix the temperature scale so that it is consistant over time

[tsundvoll]

When converting from .fff to .jpg:

Have the same color represent the same temperature each time, instead of it being dynamic based on the values in the image

[nicholasdalhaug]

Are we sure we want this? Note that different equipment will have different tempreatures. We might want to separate between small temperature differences in som cases, and only look at large differences in other cases.