Merge branch 'develop' into feature/abstractify

* develop:
  Updating submodules
Add SNPConstraintProducer protocol
Add SNPAttributeType protocol
Change SNPConstraintProducer description
Add ConstantType to ConstraintProducer
Remove SNPAttribute base class
Update SNPExpression class
Merge ConstraintProducer and AttributeType
Update xcodeproj
Further generalize SNPExpression
Operators overhaul
Remove usage info from SNPOperators
Merge branch 'develop' into feature/abstractify

* develop:
  Remove unnecessary SNPVersion file

Conflicts:
	Snappy.xcodeproj/project.pbxproj
Rethink expressions
Merge expression files
More abstract goodness
Rename ConstantValueType to ConstantType
Operators overhaul part two
Update SNPEdgeAttribute
Improve documentation