femtools: New generic function element_neighbour_owned
femtools: New generic function element_neighbour_owned

Add a new interface and associated specialisers for mesh_type,
scalar_field, vector_field, and tensor_field.  This allows us to check
if the neighbour of a specified element is owned by this process.
Effectively we're asking if it is a local element, or in the L1 halo.

iakovos-panourgias/fluidity,iakovos-panourgias/fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,iakovos-panourgias/fluidity,jrper/fluidity,jrper/fluidity,rjferrier/fluidity,rjferrier/fluidity,jjo31/ATHAM-Fluidity,rjferrier/fluidity,jjo31/ATHAM-Fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,rjferrier/fluidity,iakovos-panourgias/fluidity,jrper/fluidity,rjferrier/fluidity,iakovos-panourgias/fluidity,jjo31/ATHAM-Fluidity