Enable n_epochs=0 to get the initial embedding

[jondo]

This extends the range of possible n_epochs values below the current limit 11.

It is e.g. useful to examine the computed spectral embedding. Also, the resulting embedding can then be masked to fix points.

Note that for the new value range, in simplicial_set_embedding the graph data is only thinned with the n_epochs default value, otherwise spectral embedding would give UserWarning: Graph is not fully connected, spectral embedding may not work as expected (or even loop forever for n_epochs=0).

[coveralls]

Coverage remained the same at 88.899% when pulling e442bcd on jondo:enable_low_n_epochs into f86c922 on lmcinnes:master.

[lmcinnes]

Thanks; this is a useful change -- it is not something I had really looked at, but this enables the functionality you want nicely without doing any harm to anything else.

[jondo]

Thank you for merging this!