Implement pointobservation

[finsberg]

I this PR we add PointObersevation class which builds the operator $B$, such that

$$ d = B u, \qquad B \in \mathbb{R}^{n_d \times N}, \qquad B_{ij} = \phi_j(x_i), $$

as well as a new function point_obersvation_misfit which build the following functional

$$ J(u) = \frac{1}{2\sigma^2} \lVert W (B u - d) \rVert^2 . $$

Given some poinst where you have your obersverations points, the operatror B is created as

B = dolfinx_adjoint.PointObservation(V,  points)

where V is the relevant finite element space.

AI assistance

I used Claude Code (Claude Sonnet 5) to help profile, implement, and iterate on several of these changes. I reviewed, tested, and take responsibility for the final contribution.