add tutorial(s) documenting common operation features

[gidden]

I find that the tutorials are sometimes the easiest way of discovery especially via search (I regularly google pyam as I have a hard time remembering each and every feature we implement).

So I would suggest we create one or more basic tutorials which simply outline what features we do implement (and/or greatly expand our docs with examples).

This issue should try to capture any features that would be nice to include:

• normalize()
• offset()