Import just a part of an EPC file

[valentin-gauthier-geosiris]

When the user loads his EPC file, the javascript code should be able to open the epc, list the files and propose to only import objects selected by the user (with a dedicated interface).

==> It would be better if the files are analysed to determine the objets relations : when an object is checked in the view to be imported, the dependancies should be colorised in the view or automatically selected to be imported.

[valentin-gauthier-geosiris]

Partially done in PR : #9 : now EPC file can be partially imported, and object already present in the workspace are colorised to avoid import of already present objects.

But for now, the relations of the "not imported" objects of the pending EPC are not analysed.