1.0.0 release

[braingram]

Ideally after the current jwst release we could move this package to 1.0.0 (and lay out the "stable" API).

Some issues that will need to be addressed are:

• clean up unused code (see: https://github.com/spacetelescope/stpipe/pull/155)
• move documentation from jwst to stpipe
• designate public and private API
• do we want to continue supporting subprocess calls in hooks?

[braingram]

Clarify and/or fix windows support: https://github.com/spacetelescope/stpipe/issues/162