Transmock demo

[AdriaanRol]

Changes proposed in this pull request:

• It contains a new "introduction to PycQED" presentation in the docs folder. I gave this presentation together with Niels this morning.
• Updated the transmock tutorial notebook to work, it's actually quite cool :).
• contains some small changes to the transmock object, for some reason there where obvious bugs in there. I did not strive to fix all possible bugs here.

@MiguelSMoreira , @elrama- I do not know who is currently the "lead" maintainer of PycQED but it should be pretty easy to merge this as it is mostly just documentation and examples.

[MiguelSMoreira]

Everything seems good! Thank you for the work on the introduction and demos.