Use upstreamed averaging operator

[lukem12345]

Currently, the averaging operator from 0-forms to 1-forms is copied in a few places in this codebase.

CombinatorialSpaces.jl PR 97 upstreamed this operator.

We should use the upstreamed operator since it is actively tested and maintained there.

We will lose a little bit of pedagogical value from demonstrating in the Decapodes docs how to create an operator, but this distracts from the focus of this package.

A quick search reveals about 5 separate files that should be updated:

• Ice dynamics docs page
• CHT example
• CHT CUDA example
• Budyko-Sellers-Halfar example
• Budyko-Sellers-Halfar docs page

Projects that rely on Decapodes, such as the Veronis.jl lightning model or the FEED.jl electron dissipation model will benefit as well.