Resolves issue #60
The commit resolves issue #60, i.e. it enhances our python script to also generate the C++ header files for operators with mixed finite element pairs automatically from generic UFC form files. The changes are rather unintrusive. We now only distinguish in the script and the form files between elementTest and elementTrial for the test and trial spaces. The name and order for both elements is given in the elemList of the script. Due to the rather irregular placing of generated C++ files for mixed forms in the source tree, we now need to specify their placement in the outDir list.