Naming of functions to get nodes on facets

Currently, it is not obvious that `Ferrite.facets(::AbstractCell)` only returns the node ids of the vertices of each facet, hence, it could be good to rename these functions to have something like ``` facet_vertex_node_ids(c::AbstractCell) = facets(c) # Can leave old `facets` just for backwards compatibility? function facet_node_ids(c::AbstractCell) ipg = geometric_interpolation(c) return map(facetdof_indices(ipg)) do dof_ids map(i -> get_node_ids(c)[i], dof_ids) end end function facet_node_ids(c::AbstractCell, facetnr::Int) # Possible for optimization, but doesn't seem necessary ipg = geometric_interpolation(c) return map(i -> get_node_ids(c)[i], facetdof_indices(ipg)[facetnr]) end ``` Ref #1259