WIP: Add conical volume bounds

Closes #711

New conical shapes needed for Belle-2.

This relies on !777 (merged) and !778 (closed) to be moved in first, as it would otherwise create quite some rebasing headache.

It also fixes a bug (missing inline definition for PolygonBounds<>).

The new ConicalVolumeBounds come in several features, all of them are displayed as .obj files and tested with a new unit Test.

The most complex sectoral double cone :

A quick inside-outside test: