Improve descriptiveness of verbose output

[cciccole]

The verbose output could be improved. Here are some suggestions in no particular order.

1. don't complain about lack of PCA on high-dimensional data when there is not high dimensional data and thus it's not relevant.

2. be more specific about exactly what's happening. On large datasets I just see "pre-processing" early on and it can stay that way for a long time. What's it doing? The output should be specific about exactly which step is happening. For long running steps, provide incremental output. Not sure incremental is possible with nearest neighbors but that would be particularly useful.

3. note that when the TriMap settings are printed to stdout they do not include all the relevant settings. n_dims, for example, though I guess you are more conservative with this argument for now since it's also not documented and you mention it's untested.