DICOM-Atlas v0.2.6

Fixed

• CITATION.cff: change type from dataset to software and
  license from a YAML list to a plain Apache-2.0 string. Zenodo's
  cffconvert integration does not handle type: dataset or a license
  list, causing "Citation metadata load failed" on both v0.2.4 and v0.2.5.
• CITATION.cff: remove license-url (not recognised by cffconvert).

Full Changelog: v0.2.5...v0.2.6