SUSHI should report error when fixing values on root path that is sliced and fixed in the slice

[cmoesel]

This is best explained by example:

Profile: MyProcedureLabProfile
Parent: http://hl7.org/fhir/us/core/StructureDefinition/us-core-observation-lab
* category = OBSCAT#procedure

US Core slices category and fixes the Laboratory slice to laboratory. The rule * category = OBSCAT#procedure, however, implies a single allowed value (which is not laboratory). Therefore, SUSHI should report an error (after all, the IG Publisher does).

In addition, we probably should think in general about what it means for an author to fix a value on a multi-cardinality path. Should we emit a warning (even when there is no conflict w/ an existing slice) since the intent probably isn't to fix every element in the array (and slicing would be more appropriate)?