Add transform method to DiffusionMapEmbedding

[jolespin]

To transform a new dataset based on the diffusion map model. Similar to the last part here:
https://pydiffmap.readthedocs.io/en/master/usage.html

mydmap = dm.DiffusionMap.from_sklearn(n_evecs = 1, alpha = 0.5, epsilon = 'bgh', k=64)

dmap_X = mydmap.fit_transform(X)

dmap_Y = mydmap.transform(Y)