Robin/Neumann BC parameters were not space dependent

[voferreira]

Description of the problem

• The convection-radiation-flux boundary condition had function expression parameters that were not space dependent.

Description of the solution

• Made it space-dependent using the function evaluation at the quadrature point position.

How Has This Been Tested?

• Simple 2D heat flux case. The test was added to the application tests.

Documentation

• Remove disclaimer from parameters guide regarding the no longer existent limitation.

Comments

• This PR is a follow up of PR #1000 .
• There is no way to have set number of fluids = 0. This was added to the maintenance project and I will fix it in another PR.

[voferreira]

@blaisb Thanks! I thought set_time was called on update_boundary_conditions, but the if in the integration was preventing it from being called. This is fixed now. Plus I have placed the assertion for the emissivity at the right place, I believe. I also added a transient test for heat_flux = 10ty.

[blaisb]

I'll let @mivaia take a look at it and then if he's ok with it I'll merge