Add unit tests for spectrum attributes, including empty
The idea of this PR is to see that the tests fail before merging PR #273, and that they pass afterwards (I'll merge develop
into this branch).
Tests added that:
- Verify basic reading and writing of spectrum attributes.
- Verify that clients reading empty spectrum attributes get a
None
value. The string spectrum attribute is a special case as it doesn't reduce its length when written to.
Relates to issue #271 (closed), and PR #273.