Felupe

Fixed
- Fix the gradient evaluation in `NeoHookeCompressible(mu=1, lmbda=None)`.
- Fix `MaterialStrain.plot()`.

Added
- Add `umat_new, res = ConstitutiveMaterial.optimize(ux=[stretches, stresses], ps=None, bx=None)` to optimize the material parameters on given experimental data for incompressible uniaxial, biaxial and / or planar tension / compression data by `scipy.optimize.least_squares()`.
- Add initial default material parameters for the hyperelastic material model formulations.

Added
- Add `math.solve_2d(A, b, solver=np.linalg.solve, **kwargs)` to be used in `newtonrhapson(solve=solve_2d, ...)` for two-dimensional unknowns. This is useful for local Newton-iterations related to viscoelastic evolution equations inside constitutive material formulations.
- Add x- and y-offsets in `Job.plot(xoffset=0.0, yoffset=0.0)`.

Changed
- Wrap the ax-title with the parameters of the material model in `ConstitutiveMaterial.plot()`.

Fixed
- Sort array of principal values in descending order before plotting in `Scene.plot("Principal Values of ...")`. This ensures that the labels are matching user-defined arrays of principal values.

Fixed
- Set the default verbosity level to None in `newtonrhapson(verbose=None)` and `Job.evaluate(verbose=None)`. If None, this defaults to True (as before) but evaluates the environmental variable `FELUPE_VERBOSE` if present with `FELUPE_VERBOSE == "true"`. This does not ignore custom verbosity levels.

Added
- Add a method to convert a mesh to a PyVista unstructured grid `Mesh.as_pyvista(cell_type=None)`.

Changed
- Change default line-width from `1.0` to `2.0` in `ViewMesh.plot(line_width=2.0)`.
- Enforce a 3d points-array in `Mesh.as_meshio()`.

Fixed
- Fix previously ignored line-width in the plot of a mesh `ViewMesh.plot(line_width=1.0)`.
- Fix `math.tovoigt()` for one-dimensional tensors.

Fixed
- Fix `FieldContainer.evaluate.strain(fun=lambda stretch: stretch)` for custom strain-stretch callables. The `fun`-argument was previously ignored.