Add new CurlCurlSolve() to propagators

[Monoclod]

Add a new propagator called CurlCurlSolve() to find weak solutions to the curl-curl problem.

Find $\mathbf{E} \in H(\textnormal{curl},\Omega)$ such that

$$ \int_\Omega (\nabla \times \mathbf F) \cdot (\nabla \times \mathbf E)\ \textrm{d} \mathbf x - \sigma \int_\Omega \mathbf F \cdot \mathbf E\ \textrm d \mathbf x = \sum_i \int_\Omega \mathbf F \cdot \mathbf J _i\ \textrm d \mathbf x \qquad \forall \mathbf F \in H(\textnormal{curl}), $$

where $\mathbf J _i:\Omega \to \mathbb R^3$ is a real-valued vector field and $\sigma \in \mathbb R$ is a scalar.
Boundary terms from integration by parts are assumed to vanish.

The equation is discretized as

$$ \left( \mathbb C^\top \cdot \mathbb M^2 \cdot \mathbb C - \sigma \mathbb M^1 \right)\cdot \boldsymbol e^{n+1} =\sum_i \mathbb P^1 \cdot \boldsymbol J _i , $$

where $\mathbb M^1$ and $\mathbb M^2$ are objects of the struphy.feec.mass.WeightedMassOperators class, and $\mathbb P^1$ is the projector into the space $V^1_h$.

The method does not converge properly for sufficiently high spline degrees and sufficiently fine mesh grids due to the inherent ill-conditionedness of the curl-curl problem; this should be solved by proper initial conditioning.

This PR also adds tests for the new propagator.

[Monoclod]

Propagator does not seem to reach the desired convergence rate for linear splines (even if it still converges and the rate is somewhat close) while the others all seem to work fine, so for the moment the test file does not test for spline degree p = 1.