When distance_threshold is set then it is used to determine the number of clusters to cut the tree at.
Though it is to be noted that this works only when computer_full_tree=True.

* When building the tree the return_distance set to True if the distance_threshold has been set.
The distances returned is then used to calculate the number of clusters when cutting the tree.

* Test agglomerative clustering with distance_threshold passed in and compare with the different number of clusters produced with and without connectivity.

Changes to documentation to include distance_threshold

Updates to distance threshold in hierarchical clustering

* Moved the parameter check from init to fit for consistency

* Updates to tests to account for changes made above

Documentation changes based on review

* backticks for variables in docstrings

* formatting without backslashes

Test for hierarchical clustering with distance_threshold

* clusters produced are checked against the linkage tree to confirm that it matches the point where the distance exceeds the threshold set
* boundary case test when distance_threshold is equal to the distance

* Updated tests to compare clusters and number of clusters

* Allowing users to set n_clusters or distance_threshold and updated tests

* Checking the n_clusters None condition better

* Removed the necessity for n_clusters_ to be set to None and redundant checks

* Updated tests after the above changes

* Cleaned up test to compare clusters produced using n_clusters against distance_threshold

* Added and Simplified test for boundary conditions

* Updated the documentation on distance_threshold restrictions

* Doc string updates for clear information

* Removed redundant attribute n_clusters_

* Fixed tests

* Changes to FeatureAgglomeration to include distance threshold

* Added a new attribute `n_clusters_` to hold clusters computed when distance threshold is set

…tering_threshold

…tering_threshold