[a.fixSWIGWrapperBuild_i891] Avoid unnecessary rebuilding of the SWIG wrappers (Closes #891)

Merging branch 'a.fixSWIGWrapperBuild_i891'  into 'main'.

* `SwigLib.cmake` is modified so that SWIG-produced wrappers be produced once (unless dependencies are updated).  
* All SWIG-related fragments are gathered into `SwigLib.cmake`.
* The dependencies of PyCore and Fit modules are corrected.
* Example Python-scripts are installed.

Closes issue #891

See merge request !2357