Provide limited support for arbitrary units in UCUM

[brynrhodes]

UCUM supports the notion of "arbitrary units", which are units that may differ based on circumstance. For example, international units [IU] differ based on the substance.

The UCUM specification indicates that these units "are not of any specific dimension" and "are not commensurable with any other unit", so operations on these units are not allowed.

This means that if an engine is asked to perform the following, UCUM indicates it should fail:

Dalteparin.dosage() >= 10000 '[IU]/mL'

However, clinical intent in this case is fairly clear, and given that the authors have identified the substance, intent is for this comparison to be performed as expected.

To ensure this case is considered correctly:

1. The translator should issue a warning when operations involving arbitrary units are detected
2. The engine should support this comparison but only when one of the operands is a literal

[brynrhodes]

CQL specification issue proposing guidance on arbitrary units be added to the specification:

https://jira.hl7.org/browse/FHIR-52443