[Doc][Enh] PythonTask: examples of package relative import or installation from source

[JusteRaimbault]

In a python script, importing relative packages (py files in the samme directory or in a subdirectory) is a common practice, we should in the documentation give an example how to do this properly, combined with the use of resources or inputFiles. Similarly, it would be good to illustrate how to install a python package from source using install (related to #553 : this needs the hostFiles option to work to mount source files during preparation of the container).