Plancharel (#47)

The proof of Plancharel's theorem is done, except for one theorem about
convolutions that is missing from mathlib (which I didn't notice
earlier) and a silly norm-cast lemma that I couldn't figure out.
Unfortunately, I couldn't prove that L^1 intersected L^2 is a normed
space. I don't understand why, but nothing I tried worked in instance :
NormedSpace. This also means that I couldn't even state the following
lemmas (The inclusion is DenseInducing, etc.) without throwing errors.
I also updated the .tex file, so that the online version should have all
the links and the dependency graph should display the progress
correctly. (I did not test this, apart from making sure the LaTeX
compiles.)