shapes-ISSUE-194 (valueStem): stems in value sets

shapes-ISSUE-194 (valueStem): stems in value sets

http://www.w3.org/2014/data-shapes/track/issues/194

Raised by: Eric Prud'hommeaux
On product: 

The current SHACL definition for stem (see 4.5.4 sh:stem <http://w3c.github.io/data-shapes/shacl/#StemConstraintComponent>) treats them as a Parameter while ShEx uses them in value sets (see 4.4.6 Values Constraint <https://shexspec.github.io/spec/#values>). The SHACL definition use doesn't offer much value over an XSD pattern. It's pretty common for clinical value sets to require any value with starting with one of a number of base IRIs (e.g. terminologies like LOINC, SNOMED, CPT, ...).

Another diff is that ShEx value sets have exclusions which can in turn be stems (see 10 Value Sets <http://shex.io/primer/#h-value-sets>)