Iftach Sadeh / ctools

…new refitting logic, and revert back to original reference values for the unit tests

The "refit_if_failed" parameter instructs ctlike to perform a refit in case of a fit failure. Fit failures are either a stalled fit, an exhaustion of the number of iterations, or a significant difference between the number of observed and predicted events.

# Conflicts:
#	doc/source/users/tutorials/notebooks.rst

Update 'test/data/spectrum.fits' to use the new column names

# Conflicts:
#	ChangeLog
#	NEWS
#	README.md

Add tests to csbkgmodel for multi-instrument observations (#2862)

This allows setting the exclusion map without having to save the map first

This enables accessing the generated skymaps without calling 'save()' first

…parameter, and add csbkgmodel unit test