Draft for resolving issue 48

The commit
- adds component_index as keyword to IntegrandSymbols
- makes gradient and divergence form pass component_index to process_integrand
- the corresponding integrands now check the shape of grad_u resp. grad_v
  and, if it is not square, revert to extracting the corresponding component
- moves the is_implemented check from the call of HyTeGIntegrator outwards,
  so that the call to form_func is protected against out-of-bounds indices