Add support for URL ancillary variables

[DocOtak]

Adds support for the creation of string type ancillary variables of various shapes that can be attached to primary variables. The intent for now is to link to external data sets and not to, for example, methods papers or documentation.

TODO:

• Remove URI param
• editing machinery (won't work with normal fq) maybe add station/cast/[sample] indexing support?
• sort out attrs, CF might use a standard name or might use something like "cf_role", we cannot wait on this so will probably just do a "cchdo_role" attr
• exchange export support
• tests

[coveralls]

Coverage Report for CI Build 27429050749

Coverage decreased (-0.8%) to 87.998%

Details

• Coverage decreased (-0.8%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 34 coverage regressions across 1 file.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

34 previously-covered lines in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
cchdo/hydro/core.py	34	83.09%

---

Coverage Stats

Relevant Lines:	2658
Covered Lines:	2339
Line Coverage:	88.0%
Coverage Strength:	0.88 hits per line

---

💛 - Coveralls