Remove sequenceNumber from SNAPSHOT

AIXM Temporality 1.0, page 9:

| Note that for a SNAPSHOT, the correctionNumber and the
| sequenceNumber properties shall not be populated.

http://www.aixm.aero/gallery/content/public/AIXM51/AIXM%20Temporality%201.0.pdf#page=9
Merge pull request #5 from m-click/remove-sequencenumber-from-snapshot

Remove sequenceNumber from SNAPSHOT