Allow writing a NRRD record to memory. Ref #108

[bernardopericacho]

Proposed changes:

• write now supports as a parameter either a filename or a io.BytesIO object.
• refactor write method into:
  • _write_default_header_entries to write the default header entries generated by this package
  • _handle_header_fields to determine the headers based on the specified headers, data and index order
  • _write_header that used the logic from _write_default_header_entries and _handle_header_fields to write the headers
  • write used the methods above to encapsulate and reuse better the code
  • added new tests to check the functionality. Coverage still 100% after these changes

[addisonElliott]

One more thing. Can you add examples for reading and writing to memory? That's probably useful to demonstrate.

[bernardopericacho]

Thanks for the review and the feedback on this. I will apply the suggestions as soon as I can.

[bernardopericacho]

I think I addressed all the feedback if you can have another look. Thanks in advance!