Replace `dolfinx.fem.Constant` with a real function space

[jorgensd]

Currently, DOLFINx uses two UFL concepts, namely ufl.Constant and ufl.Coefficient.
The ufl.Constant is tied to a domain (dolfinx.mesh.Mesh), even if it doesn't leverage any information from it.
The primal motivation for having a domain in Constant is differentiation (ref: FEniCS/ufl#313 (comment)).
However, we don't use this in DOLFINx atm.

Pros with dolfinx.fem.Constant:

• Faster packing, as there is only one value per domain

Cons with dolfinx.fem.Constant:

• Extra input to kernels, i.e. extra code and special handling in FFCx
• Extra packing routines in DOLFINx, i.e extra code
• No clear instructions on ownership of the values in the constant. One is required to manually sync this value.

Pros with dolfinx.fem.FunctionSpace(real_element)

• Clear ownership of values
• Differentiability
• Less code in FFCx and DOLFINx for packing and passing data to kernels.
  Cons:
• Slightly more packed data (real space is equivalent to packing a DG-0 function.

[jorgensd]

To fix the packing of real spaces, one could do:

a = pack_data_of_one_constant_cell.flatten()
b =  np.lib.stride_tricks.as_strided(a, shape=(repeats, len(a)), strides=(0, a.itemsize))

This is illustrated in:
scientificcomputing/fenicsx_jax@50018d4#diff-0654a0e1d0afa67b21f86c5c6e6051b5d8214be39d641cebb86cf1fa7f1ffe1eR136-R141