[WIP] Add Robin BC for scalars

[yslan]

Support Newton cooling BC (Robin) for scalars.

In the examples/robin, we have three tests.

1. TEMPERATURE: diffusion equation with Newton cooling BC. The temperature decays at a known asymptotic rate.
2. 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.

WIP

• Need extra works to merge into next branch.