Improvements week of nov 6

[ClimbsRocks]

• [x] prediction_intervals complete
• [x] allow the user to specify just one interval
• [x] only train up the intervals that users actually ask for
• [x] figure out return values (probably a dataframe by default)
• [ ] add documentation
• [ ] add examples
• [x] add more tests?
• [ ] look into how r does this, and consider using their return format

Raghav's ask for a more basic training

• [ ] primarily, taking in the user's number of trees if they give it to us, rather than using X_test

• [x] dataframevectorizer performance improvements

• [x] we've already got an issue open for this- look there for more info.

[ClimbsRocks]

• [x] remove tensorflow as a requirement. consider removing keras as a requirement as well? that might break things

[ClimbsRocks]

• [ ] pin versions