No warning when passing nothing to the immersed kwarg of FieldBoundaryConditions

[simone-silvestri]

No description provided.

[glwagner]

This seems a little dirty, why can't people use DefaultBoundaryCondition? In other words I don't see the point of having two names for this.

[simone-silvestri]

I thought that nothing is not really a boundary condition, so it kind of seems awkward a warning that says that nothing immersed boundary condition is not supported on a non-immersed grid

A user might think that they need to provide a specific immersed boundary condition, but I am ok closing this without merging.

[glwagner]

I thought that nothing is not really a boundary condition, so it kind of seems awkward a warning that says that nothing immersed boundary condition is not supported on a non-immersed grid

A user might think that they need to provide a specific immersed boundary condition, but I am ok closing this without merging.

I guess the default is nothing, right?

The question is, why do users set the bc to nothing? What do we want users to do? Basically the people that suffer from this PR are people trying to read code, because they might be reading code that says something like

bcs = FieldBoundaryConditions(immersed=nothing)

and interpret this as "doing something" (changing a default). But actually, this is not changing the default. So the question is whether we want to encourage people to write the code above or not.