nano tutorial needs update

[jarlebring]

There is a new version of the online code from G Demesy that we base our nano-tutorial on. The tutorial still works, but you would need to go back to an old version of his code. We should update the nano-tutorial to use the new version of the code (i.e. update the simulation output as well as the modification description). In order to not have to do this again if they decide to update the code again, I propose we now provide a URL pointer to their code directly to to the current commit (not newest), i.e something like:

https://gitlab.onelab.info/doc/models/tree/be2c09ebd6c3109e6ca2a703c2e5ae54abf16cea/NonLinearEVP

[jarlebring]

In the previous commit I changed the URL-link to a specific (old version) of their code, so the tutorial works exactly as explained. It is still probably a good idea to update the tutorial to the new version of their code.

[jarlebring]

If we are anyway going through the tutorial again, we might as well also update and use https://github.com/tkonolige/PETScBinaryIO.jl instead of our own crude implementation of reading sparse PETSc-matrices. Note that you need to provide the correct kwargs to readpetsc:

Z=readpetsc("file_mat_M15.m.bin",int_type=Int32,scalar_type=ComplexF64)