Add support for customizing sample labels

[clausmith]

A user should be able to customize how the samples are labelled along an axis. For example, specifying that the plot be labelled by external_sample_id, or a lambda function.

[polyatail]

Technically, this still exists by calling, for example:

# method 1
samples._collate_metadata(label="metadata field")

# method 2
samples._collate_metadata(label=lambda metadata: "my formatter {}".format(metadata.field))
samples.plot_pca()

But I agree that it should be made more accessible. Perhaps making collate_metadata public?

[boydgreenfield]

@polyatail I’d error towards making this doable in the plot_* calls directly. Seems like exposing metadata collation is quickly getting into “expert end user” territory and I think a wider audience would like/use this functionality.