Wrijupan Bhattacharyya / ctools

Although the get_obs() method was put back, ctmodel used the dummy cube (which consisted of a single pixel) instead of a cube defined from the input parameters. This was due to an internal change of the logic, that now has been corrected for.

Without the energy margin, roundoff errors may lead to the exclusion of the energy bin in case that a single energy bin is used.

…d in observation ebounds. Also added some improvements to csspec.

…lso modified consistently, csresmap can now also take two maps (cnt and model) as input and compute the residuals directly. The algorithm parameter has been made unhidden

Conflicts:
	ChangeLog
	NEWS
	src/ctbkgcube/ctbkgcube.cpp
	test/test_ctools.sh

…vise csresmap parameters and attach response to observation if needed.

…(see #1360 for details).

…)-function of individual ctools