Include UDFs in Session 7 #10

Session 7 on xlwings currently does not have much information on User Defined Functions. This is primarily because I could not get it to work properly.

In the xlwings add-in in Excel, the UDFs would not import. The error complained about xlwings32-0.18.0.dll not being present, even though it was in the same directory as the Python installation.

Anyway, the exercises are currently more than enough to fill the time for a session, but it would be nice to have UDFs covered at some point since they are really useful.