TEMPERATURE: diffusion equation with Newton cooling BC.
The temperature decays at a known asymptotic rate.
SCALAR: isolated Poisson solver and isolated Helmholtz solver with three BCs
At tstep=0, we solve Poisson equation
At tstep=1001, we solver Helmholtz
where SCALAR01 - SCALAR03 are using Dirichlet, Neumann and Robin BC
This demonstrate how to solve extra time-independent elliptic solver.
Support Newton cooling BC (Robin) for scalars.
In the examples/robin, we have three tests.
WIP