`dolfinx.fem.assemble_scalar` is not parallel safe

[RemDelaporteMathurin]

@jorgensd realised that this is not parallel safe and would return the wrong result in parallel.

https://github.com/festim-dev/FESTIM/blob/77c97b30b7b9db90cc35754c8228b3f8f52a7f6d/festim/exports/average_surface.py#L28-L31

Would be nice to have a demonstration of this.

Instead we should have something like this:

https://github.com/scientificcomputing/scifem/blob/02f1d9b6ae0b3c394667524e65571729260e9859/src/scifem/assembly.py#L9-L22

[RemDelaporteMathurin]

We should just use scifem.assemble_scalar instead of fem.assemble_scalar