Fixed substitution.
Refactorings and documentation
Throwing error if no sort exists when checking subsorts.
Merge branch 'user-substitution' into tutorial

Conflicts:
	java-backend/src/main/java/org/kframework/backend/java/kil/Sort.java
	java-backend/src/main/java/org/kframework/backend/java/symbolic/UnboundedVariablesCollector.java
	java-backend/src/main/java/org/kframework/backend/java/symbolic/UserSubstitutionTransformer.java