Tr/rm bcs

[imreddyTeja]

• Removed SetValue from GradientC2F, DivergenceC2F, CurlC2F, UpwindBiasedProductC2F
• Removed SetGradient from InterpolateC2F and WeightedInterpolateC2F
• Removed AdvectionC2C, AdvectionF2F, FluxCorrectionF2F, and FluxCorrectionC2C operators

The step example is now merged with the advection example.

todo: maybe make some of the boundary conditons used in examples lazy

• Code follows the style guidelines OR N/A.
• Unit tests are included OR N/A.
• Code is exercised in an integration test OR N/A.
• Documentation has been added/updated OR N/A.

[akshaysridhar]

Thanks @imreddyTeja. I looked over the artifacts (and updated docstrings) in the examples/column and hybrid/plane + box sections the outcomes are reasonable. @dennisYatunin can comment on the operator_matrices.jl changes. BorisBook FCT operators are not used anywhere in ClimaAtmos.jl. FluxCorrection<> ops are also unused in ClimaAtmos.jl - the related examples (density current / rising bubble) could be written with prescribed constant diffusion to simplify them if needed.