Correct BCs with wrappers (including mixed options)

[ta440]

Currently, if any wrappers are used, the boundary conditions default to Dirichlet conditions on all fields, which surely wouldn't be correct for all cases?

In the time_discretisation.py file:
elif self.wrapper is not None: # Transfer boundary conditions onto test function space self.bcs = [DirichletBC(self.fs, bc.function_arg, bc.sub_domain) for bc in bcs]

This arose as an error when trying to use the mixed_options wrapper when simultaneously transporting multiple fields with SIQN. When doing so, we use the full mixed space, so applying the individual Dirichlet BCs is not compatible with this.