Correct typos
Add std:: to pow() and sqrt() functions.
Add documentation.
Set PSF attribute before reading PSF information.

The PSF attribute FRONT/BACK has to be set before reading the scaling parameters as they depend which parameters are read. Before, the attribute has been set after reading the scaling parameters, which led to an application of the front scaling factors to the back response (see issues #603 and #1004).
Remove theta limitation to 70 degrees.