Documentation: Doxygen page about interference functions (text and figures), UserManual (modifications of ff.tex and interferences.tex)

new layout of User Manual,
addition of Python examples in Examples/python/UserManual,
corrections of Python scripts in Demos (about Layout and MaterialManager),
modification of some 2D figures of form factors

Parallelepiped form factor removed.
Corrections of some bugs in FormFactor InfiniteLongBox

Pdf versions of contour plots for output intensity for all form factors in User Manual Appendix.
Addition of a description of Core Shell particle in User Manual Appendix.

Form factors: addition of appendix to User Manual, schematic views and missing information to Doxygen

New doxygen docs switched on, Doxyfile.in is introduced for automatic propagation of version number in the project.

- no latex output
- html output goes to BornAgain/html