Add DIGRAPHS_DeclareObsoleteSynonym

[William-Kenyon]

Fixes #735, adding DIGRAPHS_DeclareObsoleteSynonym, which behaves similarly to DeclareSynonym, but produces a warning each time the synonym is called, i.e. "xxx" is obsolete and will be removed in a future version; use "yyy" instead,.

Since the testing environment sets the warning level to 0, I had the tests temporarily set it to 1 and then restore the original value. I checked to see how other tests handled warnings, but could only find ones in grahom.tst, which didn't require changing the warning level. This could be because their warning messages come from homos.c, so bypass GAP.

This is progress towards #570, keeping DigraphDijkstra as an obsolete synonym for DigraphShortestPaths.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.48%. Comparing base (f4fc1ac) to head (8b25cee).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #955   +/-   ##
=======================================
  Coverage   97.47%   97.48%           
=======================================
  Files          51       53    +2     
  Lines       21301    21363   +62     
  Branches      639      639           
=======================================
+ Hits        20763    20825   +62     
  Misses        473      473           
  Partials       65       65           

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