Clusteval

* bug fix in the special case when no clusters are possible to detect

* check for correctness of the input parameters cluster and method.
* better error handling for hdbscan

* bugfix for hdbscan: AttributeError: 'clusteval' object has no attribute 'results'
* Added licence in setup.py

* fix for dendrogram cut in special cases

* bug Fix in case no clusters are detected

Huge structural update.
The input parameter "method" can only be used for the evaluation of methods, such as silhouette, dbindex and derivative. The clustering approach can separately be chosen using the input parameter "cluster", which can be: agglomerative, kmeans, dbscan, hdbscan.

* added input parameter: "cluster"
* input parameter "method" is for: silhouette, dbindex, derivative
* Changed input parameter minclusters to min_clust
* Changed input parameter maxclusters to max_clust
* Pep8 styling
* code refactoring
* Docstrings updated