[pyclustering.cluster.agglomerative] Ward's method for cluster merging

**Introduction**
The was a topic on **stackoverflow** referred to **pyclustering** where Ward's method was mentioned. It is a method that is used as criterion for choosing pair of clusters for merging (as well as existed in our implementation SINGLE_LINK, COMPLETE_LINK, etc.). Ward's method should be introduced to agglomerative clustering algorithm.

**Resources**
- Wiki page: https://en.wikipedia.org/wiki/Ward%27s_method
- Ward, J. H., Jr. (1963), "Hierarchical Grouping to Optimize an Objective Function", Journal of the American Statistical Association, 58, 236–244.

**Task description**
1. The method should be implemented for 'pyclustering.cluster.agglomerative' module. Existed 'type_link' (that consists of SINGLE_LINK, COMPLETE_LINK, AVERAGE_LINK, CENTROID_LINK) should be renamed to much appropriate enumerator.
2. Ward's method should be introduced in the renamed enumerator.
3. Ward's method should be implemented.
4. Unit-tests should be implemented for this criterion to 'pyclustering/cluster/tests/unit/ut_agglomerative.py'.
5. Examples should be implemented for this criterion to 'pyclustering/cluster/examples/agglomerative_examples.py'.