Document how to include calculations (math) in various contexts, like Jinja2, mako, and in the interview in a code block #568

Could cover:

• How to do math in Python - basics like addition, subtraction, division
• Working with undefined variables with showifdef() and adding a default value
• Using sum_if_defined() specifically
• Alternatives like using a module
• Keeping code variables "fresh" when values change (depends on)
• Explain how to trigger a code block before the download screen so it is defined (and alternatives like adding a need modifier to the attachment block