Update python functions to adhere to simpler standards and pre-format data

[sgosline]

We discussed some basic alterations to the python functions that are described in the doc-update branch here:
https://github.com/PNNL-CompBio/coderdata/blob/doc-update/README.md

These changes entail:

1. renaming DatasetLoader to just dataset
2. creating functions to list,download, and load for the CoderData package
3. creating functions of the new dataset object including: train_test_validate (already exists), save, types and format

This will resolve #228 and #229, which I will close as duplicates.