code: public FaunusPipeline linkIn(final String label, final String step, final String mergeWeightKey) {
          return this.link(IN, label, step, mergeWeightKey);
      }
docs: Have the elements for the named step previous project an edge to the current vertex with provided label.
  If a merge weight key is provided, then count the number of duplicate edges between the same two vertices and add a weight.
  No weight key is specified by "_" and then all duplicates are merged, but no weight is added to the resultant edge.
  
  @param step           the name of the step where the source vertices were
  @param label          the label of the edge to project
  @param mergeWeightKey the property key to use for weight
  @return the extended FaunusPipeline