Example: Before this commit the expression

\sum_i a_i (\nabla v)_i + \sum_j b_j (\nabla v)_j

was split into two accumulation terms:

1) a_k with corresponding test function (\nabla v)_k
2) b_l with corresponding test function (\nabla v)_l

Now we split into:

a_k + b_k with corresponding test function (\nabla v)_k

This is possible since we have linearity in the test function.

TODO: Jacobians are not yet working

But only when they are actually used!

As std::string::c_str() returns it.

Funnily, we were only able to calculate symmetric problems so far.

Need to use handler from pymbolic as loopy codegen handlers
are too involved for standalone usage.