GIPSL Allow set operations on many references attributes

[MarkBeB]

This PR will also resolve #331

It will now be possible to work with attributes that have a multiplicity of 0..* (many).
These attributes will be treated as collections, in the same way as mapping.mappingId or types.typeId, and will support the arrow operator ->, and thus operations like filter, sum, etc. pp.

Tests can be found here: Echtzeitsysteme/gips-tests#137

// e.g., accumulate some values over all child nodes
constraint with ContextType {
   context.variables.accumulator == mappings.patternWithContextAndChild
         -> join( parent ) 
         -> sum( element.nodes.oneChildNode.variables.valueSpecificToChild )
}

// Can be written as 
constraint with ContextType {  
  context.variables.accumulator == context.allChildNodes-> sum( element.variables.valueSpecificToChild )
}

constraint with ContextType {
  // e.g., ensure that exactly one node out of a group of nodes 'somewhere else' is assigned to the context node
  context.referenceToAnotherNode.someMoreNodes -> sum ( element.variables.assignedTo == context.id ) == 1
}