hampyx should set the default exe_path and xml_path automatically

[luizfelippesr]

I think hampyx should be aware of the location where hamx is installed.

Likewise, when no xml file is specified, it would make sense that hampyx would use the 'params_template.xml' file.

(This issue was triggered by discussion in the IMAGINE slack space)

[luizfelippesr]

If cmake could generate a file containing the installation prefix chosen by the user, this could be straightforwardly used by the python package to locate (the default path of) hamx.

It should still be possible for the user to override this, if they want to use a different version of hamx on their system.