m20-ecam-assemble brightness matching should be optional

[sschmaus]

Since the full quality, unstretched images became available for M20, the brightness matching became unnecessary for most Engineering Camera images.

For images with non-standard tiling structures, the algorithm also produces undesirable results, like here for FLF_0923_0748887303_658ECM_N0450000FHAZ00203

I temporarily disabled the color matching in the code and assembled the image again which yields this basically perfect result:

I recommend to make it the default behaviour that brightness matching is disabled. Maybe it can be re enabled when an additional flag is specified.

[kmgill]

Added -n option for m20-ecam-assemble to skip brightness matching

[sschmaus]

Appreciate it, thanks for the change!