Fix for issue #16
Fix for issue #16

lambda got updated to lambda_val while fixing issues with numpy's structured array access; update this accordingly.
c-north/hdbscan,brunoalano/hdbscan,scikit-learn-contrib/hdbscan,lmcinnes/hdbscan,scikit-learn-contrib/hdbscan,lmcinnes/hdbscan,brunoalano/hdbscan