[firedrake] local stuff in UFL

Dear Firedrakers, With Toyo Sadare we are looking at implementing a new type of lumping scheme to remove the spectral gap from the RT2-DG1 (Norwegian numbering) spaces for linear shallow water on quadrilaterals. To implement this we need to be able to write some stuff in local coordinates instead of global, including defining coefficients that are functions of local coordinate (stuff like xi_1(1-xi_1) etc.). What is the best way to do this? all the best --cjc