- complex case is not handled yet by compile_symbol (must check if the expression is complex using sympy)
- second derivatives
- block tests are ok when created using Curl, Div, etc => glt/test_1d_block_1 is not working
- for the moment, operators dx, dy and dz are not modified by gelatize function.
- mapping
- coordinates system
- make the degree a free symbol in mass, stiffness, advection glt symbols