Refactoring derham.Derham and adding Tutorial 09 on the discrete deRham sequence

• Moved fields.Field to Derham.Field as an inner class. A field can be created by calling the factory function Derham.create_field(). The file fields.py has been deleted.
• Added Tutorial 09 on the discrete deRham sequence
• Field initialization: the init/comps dict does not need to feature ALL variable names of a species, but only those that are actually initialized
• Delete Derham.send_ghost_regions, has been replaced by StencilVector.exchange_assembly_data some time ago
• Replace Derham.E by Derham.extraction_ops
• Replace Derham.B by Derham.boundary_ops
• Replace Derham.spaces_dict by Derham.space_to_form
• Delete Derham.V and Derham.forms_dict
• Change Derham.V to Derham.Vnames
• Replace quad_order by nquad
• Show correct picture in Stencil Tutorial 08