Introduces NewTypes for Trial- and TestSpace such that mypy can check they are not confused.
Had to change FunctionSpace from Protocol to ABC.
Comparisons between test and trial spaces now trigger mypy warnings because we are comparing objects of different types. These warnings must be ignored which is a bit nasty but I think worth overall.
Fixed typing of tests.
This exercise revealed a few mix-ups of trial/test in the tests (mypy told me).