Small fix to handling of unary minus

Now adds an implicit 0.0 rather than attaching minus
to the next primary expression. Trouble arose in
expressions like

-x^2

which would be interpreted as (-x)^2. Now interpreted
as

0 - x^2

Problems could arise in an expression like

x * -y

so this should be checked for.
Adding test of Laplacian inversion

For now choosing solutions which satisfy the boundary
conditions, rather than pass evolving boundary
conditions to the Laplacian solver.

Since Dirichlet boundary conditions imposed on cell
centre, rather than mid-point, currently get
first order convergence rate.