feat: new `separate_header` kwarg in `write`

Package already supported separate header/data for `read`; these
changes update `write` to permit creation of same.

Also some pep8 fixes, and final exception raise now uses `options`
fix: now works for all options

Needed to export raw data writing code to `_write_data` subfunc
for proper functioning.
doc: update comments and name to AUTHORS
style: place `separate_header` last in `write` argument list

This ensures backward compatibility with legacy code which did not
specify the `options` kwarg, instead just providing a dict as 3rd
argument.
Merge pull request #1 from JDWarner/separate_header_write_option

Changes by Josh Werner to to allow the write function to create separate .nhdr (header) and .nrrd (raw data) files.