Move and edit variable expressions section

After some thought I decided it would be worthwhile to have
a section dedicated to describing dependency injection since
that's rather central to the simulation framework.
I put it at the end just before the API section.
The content is an edited/expanded version of that first written
by @daradib.
Update SeriesWrapper docstring in simulation framework
Name of decorated sim functions default to function name

Use the function name for tables, table sources, columns, and
injectables in decorated functions if no name is provided.
Add tests for decorated sim function defaults
Update docs for decorated sim function defaults