Add option to split data into train/test/validate sets

[martham93]

The PR to address the enhancement outlined in issue 147 introduces the option to split data into added train/test/validate of user specified sizes, this code still keeps the previous label-maker default of.8/.2 train/test split.

cc @wronk , @drewbo

[wronk]

Suggested some improvements. Main one is around allowing an arbitrary number of splits rather than just 2 or 3. I think we'd want to let people split things as they see fit, but lmk what you think.

[wronk]

@martham93, can you also update/add the relevant params to the docs? You can just follow the format of the other params

Here's the relevant page

Then you should be able to run make html from the docs/ directory to generate the new documentation.

[wronk]

@martham93, minor changes to make, but looks good overall. Good work

@drewbo, want to have a quick once over whenever you're back?

[wronk]

LGTM

[martham93]

is it okay if I merge this @drewbo @wronk ?