Python's own enum works fine for the simple values that expenseAllocation
uses, but it won't work as well for metadata like taxImplication where
many of the values aren't legal Python identifiers. Introduce our own
MetadataEnum class with the necessary functionality, and switch to that.
This is the simplest version of a common validation we're going to do:
make sure that a particular piece of metadata has one of a set of
values.
This checker needs some bounds checking but I wanted to err on the
side of committing this early because it introduces so much base
infrastructure.
