Add convective heat flux BC to heat conduction physics

[GiudGiud]

Motivation

To couple solid domains to fluid ones, and not resort to creating a BC on the side of the heat conduction physics, we should be able to create a convective boundary condition on a boundary of a heat conduction physics

Design

One more BC in the heat conduction physics. Add to FV and CG since the object already exists

Impact

More flexibility in setting up Physics + component simulations