Issue 2636 create PracticeProblem schema

[lucy-kind]

Create PracticeProblem type, and add hasPart property to existing Quiz and Course types with value of PracticeProblem.

For #2636

[danbri]

I've fixed some minor errors in the first sections that introduce new terms.

For the last two sections, I am confused. It looks like you're defining a new term :hasPart, but this already exists in Schema.org, and already is expected between any kinds of creative works. I think you can deleted these:

:hasPart a rdf:Property ;
    rdfs:label "hasPart" ;
    :category "issue-2636" ;
    :domainIncludes :PracticeProblem ;
    :isPartOf <http://pending.schema.org> ;
    :rangeIncludes :Question ;
    :source <https://github.com/schemaorg/schemaorg/issues/2636> ;
    rdfs:comment "Designates that it contains a creative work." .

:hasPart a rdf:Property ;
    rdfs:label "hasPart" ;
    :category "issue-2636" ;
    :domainIncludes :Quiz,
        :Course ;
    :isPartOf <http://pending.schema.org> ;
    :rangeIncludes :PracticeProblem ;
    :source <https://github.com/schemaorg/schemaorg/issues/2636> ;
    rdfs:comment "Designates that it contains a creative work." .```

In general, Schema.org definitions are additive. You have to consider any assertions you make in data/ext/pending/*.ttl files as being overlaid on top of what we already have in data/schema.ttl core definitions. So here you're just attaching more properties to hasPart that will just collide with the existing rdfs:comment etc. Or was the intent to somehow highlight /hasPart as being particularly relevant to /Question?

[lucy-kind]

hasPart ttl code removed; also added examples file!