Clusteval

* fix in case silscore is empty

* `sklearn` changed into `scikit-learn` in both requirements and setup file.

* Parameter `savefig `can now be used to store figures in `plot`, `scatter `and `dendrogram`

Example:

from clusteval import clusteval
X, y = make_blobs(n_samples=600, centers=[[1, 1], [-1, -1], [1, -1]], cluster_std=0.4, random_state=0)
ce = clusteval(cluster='agglomerative', evaluate='derivative')
results = ce.fit(X)

ce.plot(savefig={'fname':'clusteval_plot.png'})
ce.scatter(X, dot_size=100, savefig={'fname':'clusteval_scatter.png'})
ce.dendrogram(savefig={'fname':'clusteval_dendrogram.png'})

* added DOI

* Input parameter "method" is changed into "evaluate"
* Added docstrings
* Some code refactoring and cleaning

* improved default parameters for dbscan to detect clusters of various densities