rename geo filename square to circle
Fixes #222: Implement projector on the tangent line or plane
 - added the keyword `pT()` that gives access in formulation to the projector
 - added a test in 2D and 3D 
 - TODO: needs to be implemented for high order geometries

@vincentchabannes: again (as with `norm2()`) it seems that the gain is significant in 3D