Alert users about limitations of RobinBC function

[jbrzensk]

The RobinBC function, found in both the MATLAB/Octave and C++ implementations, is very strict regarding the Robin-type boundaries it can solve.

The key point here is that it employs "a" type of Robin boundary condition, not "any" type of condition.

I think:

• A note should be added to the robinBC.m and C++ header file explaining the specific implementation
• The MOLE documentation should also describe the behavior, making it more explicit what robinBC solves, rather than how it currently is.
• In MATLAB/Octave, documentation should encourage the use of the newer boundary condition operator mixedBC.