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.

FESTIM/festim/exports/average_surface.py

Lines 28 to 31 in 77c97b3

	self.value = fem.assemble_scalar(
	fem.form(self.field.solution * ds(self.surface.id))
	) / fem.assemble_scalar(fem.form(1 * ds(self.surface.id)))
	self.data.append(self.value)

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