Catch up with Firedrake updates

[jwallwork23]

Closes #61. Take 2 of #62.

Okay there ended up being quite a few ways we had got out of date with Firedrake... apologies for the mega PR.

Summary:

• Fix import paths.
• Use Cofunction and FunctionSpace.dual().
• Update norm and errornorm to accept Cofunctions.
• Update project to accept Cofunctions.
• To enable the above two, I needed to introduce function2cofunction and cofunction2function utilities.
• Use y = Function(FunctionSpace(mesh, "R", 0)).assign(x) rather than y = Constant(x).
• Update tests accordingly.

[jwallwork23]

Oh and the Cofunction changes also made me realise a problem with the error estimation code... The flux terms were summarised by a Function which came from a linear solve, whereas the cell terms came from assembling a 1-form, which is a Cofunction. After changing the latter to reflect the former, I found a load of bugs in the error estimation testing. Hopefully, it gives better results now :)

[jwallwork23]

I'm gonna go ahead and get this merged as I'm confident it meets the requests. Thanks again for the review @stephankramer!