Add week, topic and slidenumbers

[dwoiwode]

As proposed in Issue #27, this PR reimplements a simple footer with information about the author, the title and "slideidentifier" consisting of the weeknumber, topicnumber and slidenumber. As the week/topicnumber cannot be parsed from the file/foldername (at least I do not know how to this), two new commands are introduced to set this values:

\week{<number of week>}
\topicnumber{<number of topic>}

The pdf for week 2, topic 1 is added as an example:

The main files for weeks 7 and 9 were not updated due to a different main-structure used in this weeks. This will leave the default value=0 for the week/topic.

[larskotthoff]

Do we need this? Seems that this adds one more point of failure for slides to go wrong.

[mlindauer]

@dwoiwode Thanks for the PR.

Having the slide numbers is nice (as discussed before), but adding the week and topic ID makes it harder to add new modules since one would need to change many others accordingly. (This is already troublesome with the folder structure.)