Add nitf (#294)

* Add NITF.

* Implemented license on NITF

* Updated field namings, added some doc strings.

* PR comments

* Added if has_mask on image_data_field (to confirm); change to _sizeof operator.

* Apply suggestions from code review

Apply suggestions from PR.

Co-authored-by: Petr Pučil <[email protected]>

* Added more description.

* Update NITF with more descriptive IDs than original from spec.

* Update image/nitf.ksy

Co-authored-by: Petr Pučil <[email protected]>

Co-authored-by: Douglas Goddard <[email protected]>
Co-authored-by: Ryan Speers <[email protected]>
Co-authored-by: Petr Pučil <[email protected]>