add topology optimization tutorial

[WenjieYao]

Somehow I can't create a preview on my computer, so there might be some small mistakes needed to be fixed. @stevengj @fverdugo

[fverdugo]

Hi @WenjieYao Thanks a lot for writing this!

I have built the html version of the tutorial and it has several minor typos regarding the rendering of equations etc. Try please to generate the pages on your laptop and fix the small issues (see below).

I have also added the new tutorial to the test suite (It is is failing). Try to run it locally to fix the errors. The tutorial is run as any other .jl file.

Steps to generate the html pages locally

Here you have some info. But, to summarise, these are the steps:

$ cd /your/path/to/Tutorials # Replace this with your correct path
$ julia --project=.
(Tutorials) pkg> instantiate
(Tutorials) pkg> build
julia> exit()
$ julia --project=docs
julia> using Revise # I assume that you have Revise in your global environment
(docs) pkg> dev .
julia> include("docs/make.jl")  # Open the generated web in docs/build/index.html and correct errors
julia> include("docs/make.jl") # Refresh you browser and fix more errors 
julia> include("docs/make.jl") # Refresh you browser and fix more errors 

[stevengj]

Suggested change

	# * How to apply adjoint method in Gridap
	# * How to apply the adjoint method for sensitivity analysis in Gridap

[stevengj]

Suggested change

	# We recommend you first read the [Electromagnetic scattering tutorial](https://gridap.github.io/Tutorials/dev/pages/t012_emscatter/#Tutorial-12:-Electromagnetic-scattering-in-2D-1) to make sure you understand the following points:
	# We recommend that you first read the [Electromagnetic scattering tutorial](https://gridap.github.io/Tutorials/dev/pages/t012_emscatter/#Tutorial-12:-Electromagnetic-scattering-in-2D-1) to make sure you understand the following points:

[stevengj]

Suggested change

	# * How to formulate the weak form for a scalar time-harmonic electromagnetic problem
	# * How to formulate the weak form for a 2d time-harmonic electromagnetic problem (a scalar Helmholtz equation)

[WenjieYao]

I have fixed the typos in equations and also adopted the comments from @stevengj .
@fverdugo @raelch

[WenjieYao]

@fverdugo It is failing now mainly because we need to add Zygote, ChainRulesCore and NLopt packages to the environment. I suspect that this should be a few lines added to the Project.toml file but I don't know what I should add exactly...

[fverdugo]

Thanks @WenjieYao @stevengj for this very nice new tutorial!