Period expressions

[darylwright]

Summary

Implement period expressions as described in the project roadmap documentation:

Period expressions use smart dates to create a new date type that denotes a range between two different
points of time. This also includes 'Q' syntax that denotes a quarter in a year (e.g. q4, 2009Q1). There
are also a range of period keywords used that are similar in function to those in smart dates. All of
these need to be implemented. Additionally, weekday and month names are part of some period expressions
and need to be accounted for as well.
Documentation

This issue requires completion of #15.

Other Information

No response

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