adds unit comparison tests

[raleigh-g-thompson]

fixes #51

[cmoesel]

This looks good and I learned some things that I hadn't realized the spec says. Thanks!

@brynrhodes - I think there are a few opportunities for clarification around these topics in the spec. I've filed Jira issues for them:

FHIR-53172: Comparisons Between Calendar and Definite Duration Quantities may affect this PR, as the spec is not clear about how "not comparable" should be implemented.

FHIR-53173: Comparison of Durations with Months, Days, and Years shouldn't affect this PR because I think the spec is clear enough in some parts, but it still needs clarification for consistency in other parts.

I am approving this PR because I suspect its interpretation of "not comparable" as resulting in null is likely correct, but I'd suggest we get confirmation from @brynrhodes before merging.

[alexzautke]

@brynrhodes I'm currently fixing related issues in the .NET engine. Could you comment on the current status of these tests?

[alexzautke]

@brynrhodes Do we also have related tests for date arithmetic?