docs: instantiate main env separately

[simonbyrne]

No description provided.

[ali-ramadhan]

Thank you @simonbyrne!

@glwagner I tried to fix all_*_nodes so that we can set! reduced fields (which model.free_surface now is) but not sure if [] is the best solution.

If tests pass we should merge and tag v0.54.1.

[glwagner]

Thank you @simonbyrne!

@glwagner I tried to fix all_*_nodes so that we can set! reduced fields (which model.free_surface now is) but not sure if [] is the best solution.

If tests pass we should merge and tag v0.54.1.

Looks like we need something that has length 1.

Thinking about this more I think we actually probably want a new set! function for ReducedField so that users can use a two-argument function, rather than a three-argument function with a meaningless third argument.

For the sake of this PR though I'll just put a band-aid in if that's ok.