Correctly parse LOWER_ROW edge weight format

[leonlan]

Closes #134.

• I incorrectly assumed LOWER_ROW matrices must be given in strict triangular form, but the values may be spread arbitrarily across lines as long as it matches the expected number of elements for a triangular matrix. Not strictly mentioned as TSPLIB95 specification in the Reinelt 1991 paper, but LKH3's parser also handles these cases.
• In Bug in distance calculation from flattened matrix #40, the Eilon instance was incorrectly assumed to use a different format based on the paper, and a special case was added to handle it. This was wrong: CVRPLIB treats it as a row-wise, lower triangular matrix, which is now handled correctly without special casing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5d8a8ca) to head (7cb9794).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #135   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           13        13           
  Lines          230       215   -15     
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- Hits           230       215   -15     

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[N-Wouda]

LGTM