Code/tutorial for generating master Excel sheets (Issue #51)

Two main additions:

1. Function in prepost (write_master) to write master Excel file using Pandas
2. Tutorial and associated data in docs/tutorials to show new users how to do it

merged

mentioned in commit e9a1ff17

mentioned in issue #51 (closed)

Totally Awesome! Thanks!

I've had some very minor nitpicking details but since this merge request is already merged I just went ahead and made the modifications.