Assume all modules are in pythonpath

[ircwaves]

@Applied-GeoSolutions/ags -- This makes a change to multitemporal that drops the feature of importing of 'module' from a directory pointed to by a config item 'path', like this:

        {
            "name"   : "ccc_ndviwins",
            "module" : "wcc",
            "path"   : '/tmp/random_code/',
            "inputs" : ["ndvi"],
            "output" : true
        }

and replaces it with the behavior that the 'module' config of a multitemporal step should either be:

1. a built-in module of multitemporal -- (i.e. 'module': 'fusion2' loads multitemporal.bin.fusion2)
2. a multitemporal-conformant module that is importable (i.e. 'module': 'optis.wcc' loads that module explicitly)

Note that this is backward compatible for built-in modules, and also includes the possibility of explicitly importing them as 'module': 'multitemporal.bin.fusion2')