add script for tuning demixing

Closes RAP-90, script: tune_demixing_parameters.py

python tune_demixing_parameters.py --MS '/some/path/data/L99*MS' --sky_model /path/to/A-team-model.txt

Will examine all patches in given sky model, and output witch patches to actually use in demixing.

Further tuning with number of iterations, frequency resolution is being developed.

Requirements: LINC, lsmtool, DP3