Changes to Laplace inversion boundary conditions

o Changed Dirichlet boundary condition to apply half-way between
  the bulk points and boundary points.

o Removed redundant boundary conditions in serial tridiagonal ("tri") solver.
  All boundary conditions for FFT-based solvers now set in invert_laplace.cxx

The following tests now fail:

1. test-laplace   since boundaries changed, not surprising

2. drift-instability   the Zeff = 2 case, which is particularly sensitive
   to small changes. The new solution is closer to the analytic value than
   the reference solution.
   -> CHANGED to the new reference values

3. interchange-instability  The R=10 case also fails because the new solution
   is closer to the analytic than the reference value.
   -> CHANGED to new reference values
Hasegawa-Wakatani model verified for Dirichlet boundaries

Converging at second order with sources on
the evolving vorticity and density equations,
and on the potential Laplacian inversion.