improve treatment of Open-Boundaries in Velocity-Potential calculation:  - use non zero matrix element across OB and solve for Phi 1 grid point    outside OB (i.e, where tracer OBCS are applies)  - global inbalance of inFlow/outFlow across OB: instead of modifying    velocity across OB, spread it uniformly along the OB in the RHS.
improve treatment of Open-Boundaries in Velocity-Potential calculation:
 - use non zero matrix element across OB and solve for Phi 1 grid point
   outside OB (i.e, where tracer OBCS are applies)
 - global inbalance of inFlow/outFlow across OB: instead of modifying
   velocity across OB, spread it uniformly along the OB in the RHS.

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