Add notion of Neumann BC sign to FEVariablesBase
Add notion of Neumann BC sign to FEVariablesBase

Depending on the Physics, the Neumann boundary condition term in the
weak form could have a positive or negative sign. For example, when
written in the form M\dot{u} = F(u), the Heat equation generates
a '+' when integrated by parts, while Maxwell's equations generate
a '-'. So, we'll let the Physics declare whether it should be '+'
or '-' for that variable. This can then be used by the boundary
condition evaluation when evaluating the Neumann boundary conditions.

nicholasmalaya/grins,nicholasmalaya/grins,nicholasmalaya/grins,nicholasmalaya/grins