start to implement brokenpowerlaw
Merge branch 'integration' into 1009-add_broken_plaw

Conflicts:
	inst/lat/test/data/p7v6/crab_model.xml
Review broken power law class.
first attempt for mc method
small change in mc method
Do not fix MC energies to 0.

The energies were all set to 0 in the mc() method. The respective statement has been removed so that MC energies are returned correctly.
Correct parameter gradient computation.

There were two bugs in the parameter gradient computation.

First, the wrong index was used to compute the break value gradient.

Second, the index gradients were both computed, irrespectively of the energy. However, if the energy is smaller than the break energy, only the first index gradient should be computed while the second will be 0. If the energy is larger, the first index gradient should be 0 and only the second should be computed.

Both bugs have been fixed.
Merge branch '1009-add_broken_plaw' into integration

Conflicts:
	doc/source/user_manual/modules/model.rst
Add help target (see #1048)
Update
Rename covar to curvature.

The curvature matrix, which is equal to one-half time the Hessian matrix, is evaluated and not the covariance matrix. To make this clean, all covar terms have been renamed to curvature.
Add optimizer function Python interface.
Correctly implement invert() method.
Define nested likelihood class.