Fix argument block-size in Expressions

[jorgensd]

Currently if an Expression gets a blocked test or trial function, we do not take into account this block size when accessing the underlying tabulate kernel.

The added test segfaults on the main branch

Background
A testfunction in a blocked space (say for instance bs=3) will be evaluated as
(v, 0, 0), (0,v,0), (0,0,v) in our expression kernels, where v is the basis function evaluated at a given point.

Example

import numpy as np
from mpi4py import MPI
import dolfinx
import ufl
mesh = dolfinx.mesh.create_unit_square(MPI.COMM_SELF, 1, 1, cell_type=dolfinx.mesh.CellType.quadrilateral)
value_shape = (3,)
V = dolfinx.fem.functionspace(mesh, ("Lagrange", 2, value_shape))
v = ufl.TestFunction(V)

points = np.array([[0.513, 0.317]], dtype=mesh.geometry.x.dtype)
expr = dolfinx.fem.Expression(v, points)

values = expr.eval(mesh, np.array([0], dtype=np.int32))
print(values)

should yield

[[-0.00316522  0.          0.          0.00333421  0.          0.
   0.00146907  0.          0.         -0.0015475   0.          0.
   0.24980901  0.          0.         -0.01096585  0.          0.
   0.01155129  0.          0.         -0.11594357  0.          0.
   0.86545855  0.          0.          0.         -0.00316522  0.
   0.          0.00333421  0.          0.          0.00146907  0.
   0.         -0.0015475   0.          0.          0.24980901  0.
   0.         -0.01096585  0.          0.          0.01155129  0.
   0.         -0.11594357  0.          0.          0.86545855  0.
   0.          0.         -0.00316522  0.          0.          0.00333421
   0.          0.          0.00146907  0.          0.         -0.0015475
   0.          0.          0.24980901  0.          0.         -0.01096585
   0.          0.          0.01155129  0.          0.         -0.11594357
   0.          0.          0.86545855]]

while main yields

[[-0.00316522  0.          0.          0.00333421  0.          0.
   0.00146907  0.          0.         -0.0015475   0.          0.
   0.24980901  0.          0.         -0.01096585  0.          0.
   0.01155129  0.          0.         -0.11594357  0.          0.
   0.86545855  0.          0.          0.          0.          0.
   0.          0.          0.          0.          0.          0.
   0.          0.          0.          0.          0.          0.
   0.          0.          0.          0.          0.          0.
   0.          0.          0.          0.          0.          0.
   0.          0.          0.          0.          0.          0.
   0.          0.          0.          0.          0.          0.
   0.          0.          0.          0.          0.          0.
   0.          0.          0.          0.          0.          0.
   0.          0.          0.        ]]

when it doesn't segfault

[michalhabera]

Good catch!