Better support for using Operator Aliases

[jpfairbanks]

The goal here is to use familiar Div, Grad, Curl, Lapl operators in their familiar role in vector calculus. What support do we need in the Decapodes language for that? Probably and alias operator like :=

oscillator = @decapode begin
  X::Form0
  V::Form0
  div := ⋆⋅d
  
  ∂ₜ(X) == V
  ∂ₜ(V) == -k(X) + div(V)
end

Then we can update all the docs examples to use these operators when appropriate. We can't uniformly introduce a exterior calculus definitions of div, grad, curl because of things like how gradient of Pressure should be dP but gradient of concentration should be ⋆dC. Unless we can dispatch on expected return type in the type inference code, I think that we can just introduce a manual aliasing operator into the decapode macro, and then have local aliases in each example.

[lukem12345]

Unless we can dispatch on expected return type in the type inference code,

The way I would do this without additional parsing code would be to deduce the definition of vector calculus operators using infer_types! and resolve_overloads! like we do for ⋆and d now.

e.g.

oscillator = @decapode begin
  X::Form0
  V::Form0
  
  ∂ₜ(X) == V
  ∂ₜ(V) == -k(X) + div(V)
end

osc2D = infer_types!(resolve_overloads!(oscillator))

@test osc2D == @decapode begin
  X::Form0
  V::Form0
  
  ∂ₜ(X) == V
  ∂ₜ(V) == -k(X) + div_0(V)
end |> infer_types! |> resolve_overloads!

, where div_0 is defined like ⋆⋅d internally.

I suppose that this would not work if in one 2D Decapode, the result of div of a 0Form should be a ⋆⋅d, while it should be d in another. Not sure of a pair of examples of this at the moment.

[jpfairbanks]

I think it is probably fine to both update the parser code to allow definitions of new operators as composites of operators and to add a vector_expand!(d) which will use this table for Vector to Exterior Calculus

from the wikipedia page for exterior derivative. https://en.wikipedia.org/wiki/Exterior_derivative

We could then use a cancel_isos!(d) to aggressively eliminate combinations like sharp of flat, flat of sharp, star of inv(star) and inv(star) of star.

[lukem12345]

Although PR #142 addresses some of these concerns, it does not One can consider that PR as providing reasonable defaults for div, grad, and curl. So I am going to work on this PR next. Changes to the Decapode macro will let users change those defaults. For now, I will only consider single-input-single-output aliases. Aliases for e.g. the advection operator will require rewrites that are more in-depth than single op1 renaming.

Further, carrying around these operators throughout the lifetime of a Decapode object will best be handled via nested Decapodes. This will likely require Decapodes which contain DecaExprs as certain attributes.

Probably and alias operator like :=

Going off of the selection provided by the "Punctuation" section of the manual, I think that -> is the best fit for this feature. These operator aliases are like anonymous functions. Hopefully this is familiar to people used to Julia syntax. (Parsing \lambda like in Python would also work and be easy, but would be extremely confusing adjacent to other Greek letters being used as physical quantities, like wavelength.) The single = is also worth consideration.