Enhancement for paper: Refactor src, Write docs, Refactor package IO

[zjwegert]

Rather minor but I think we could improve the file structure in src.

src/
   Optimisers/...
   Utilities.jl
   Advection.jl
   VelocityExtension.jl
   Solvers.jl
   ChainRules.jl (*)
   GridapExtensions.jl (**)
   PackageName.jl (***)

The main change to the above is moving MaterialInterpolation.jl and Benchmarks.jl into Utilities.jl.

Note
*: I think we could rename this to be more clear to people who don't typically use AD. @JordiManyer?
**: To be moved at some point
***: Need to decide name

[JordiManyer]

*: In the end that file contains most of what we need to define the optimization problem itself, so I would probably choose a name that goes in that direction.

***: I would probably opt for either something Gridap-like (i.e GridapTopOpt.jl or similar) or something totally generic, like TopologyOptimization.jl (which is surprisingly free).

[zjwegert]

*: We could go with PDEConstrainedFunctionals.jl or something. I'm not that keen on that name though...

***: OK here is a short list:

• GridapTopOpt.jl
• GridapLSTO.jl
• LevelSetTopOpt.jl
• TopologyOptimisation.jl or TopologyOptimisation.jl
  I'll have a think about it.

EDIT: I think I like LevelSetTopOpt.jl. It avoids "optimisation" where we have US/UK spelling issues with the z/s, and the level set aspect is important. I think GridapLSTO.jl is a close runner up but it isn't quite explicit enough I think.

[zjwegert]

FYI @JordiManyer, I'm working on this in the src_refactor_and_docs branch. I'll make a PR to discuss once I've setup Documenter.jl.

[zjwegert]

Still on-going.