Add method for subsetting

[ezmiller]

This PR sets up the column api's subsetting methods.

The PR does the following:

• Exports the tech.v3.dataset.column/select method to the column API;
• Adds a slice method to accompany select;
• Updates the column_exploration doc with some stuff about these methods.

[ezmiller]

@genmeblog I'm merging this so that I can keep going, but you can still comment on the changeset here, and I can circle back if necessary.