Add new tests

[tuxette]

• [x] consistency with hclust in case the objects are properly ordered
• [x] check that sum of heights is indeed the intra-cluster inertia
• [x] check that mode="within-disp" and mode="total-disp" return a dendrogram for which the final height is the total inertia

[tuxette]

• [X] tests for correct + plot
• [x] tests for cutree (see repository and examples 'pierre')