expose `format_data` to user via `helpers`

[jeremymanning]

The format_data function is useful! Let's expose it to users via hyp.tools.as_np (with PPCA='False' by default).

[jeremymanning]

hey @andrewheusser-- I implemented this change in this branch: https://github.com/ContextLab/hypertools/tree/expose_format_data

could you please:

• take a look
• add a doc string so that it appears in our API documentation
• if the changes look good, pull the branch into master
• update the version number (minor version += 1) and push to pip

[andrewheusser]

should be good! see #168