Implement "units" grammar

[markkramerus]

Our documentation describes using the word "units" to clarify that when fixing a code or binding to a Quantity, you are actually fixing or binding the units of that quantity. Functionally, the word units has zero impact; it can be ignored. Nonetheless, it is part of the grammar that needs to be implemented.

Here's the excerpt from the doc:

Quantity
In addition to having a quantitative value, a FHIR Quantity has a coded value that is interpreted as the units of measure. As such, a Quantity can be bound to a value set or assigned a coded value. The shorthand is:

• {Quantity type} = {system}#{code} "{display text}"

Although this appears the quantity is being set to a coded value, it is legal.

🚧 To make this a bit more intuitive, FSH allows you to use the word units, as follows:

• {Quantity type} units = {system}#{code} "{display text}"

🚧 and for binding:

• {Quantity type} units from {value set} ({strength})

Note: Use of the word units is suggested for clarity, but is optional.

Examples:

Set the units of the valueQuantity of an Observation to millimeters (assuming UCUM has been defined as an alias for http://unitsofmeasure.org):

• valueQuantity = UCUM#mm "millimeters"

🚧 Alternate syntax for the same operation (addition of ‘units’):

• valueQuantity units = UCUM#mm "millimeters"

🚧 Bind a value set to the units of a Quantity (using alternate syntax):

• valueQuantity units from http://hl7.org/fhir/ValueSet/distance-units