Automatically generate package and add code tree

[nathanshammah]

Package Tree

A tree of the mitiq/mitiq package contents is added in the readme file. However, this was done "by hand". It may be nice to:

• automatically generate it
• add information in the contributing to mitiq how-to (#109) about the fact that such tree needs to be regenerated and updated in the readme once the package is updated in structure.

To do such thing, I think in linux one has the tree command. I haven't found one in Mac yet, but maybe there is one that is platform agnostic. However the tree could be updated only when there is a release, adding that info in the readme printout, to simplify code development.

Code Tree

It can be very useful (for sldes, etc.) and beautiful (to include an image of it in the documentation) to add a code tree. It does provide an intuitive way of how the library has developed in scope. I can copy the script from QuTiP's and pennylane.

[rmlarose]

I think this should be closed as #167 was closed. Please reopen if this isn't the case.