Increase rank of IsIsomorphic for nauty

[MeikeWeiss]

When nauty and grape were loaded, the IsIsomorphic method was used for grape because it has a higher rank. Since nauty is the better choice because it is faster, we increased the rank.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.34%. Comparing base (0810b52) to head (d48623c).
Report is 21 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
- Coverage   76.35%   76.34%   -0.01%     
==========================================
  Files          61       63       +2     
  Lines       16210    16382     +172     
==========================================
+ Hits        12377    12507     +130     
- Misses       3833     3875      +42     

Files with missing lines	Coverage Δ
gap/PolygonalComplexes/graphs.gi	93.63% <100.00%> (-1.29%)	⬇️

... and 7 files with indirect coverage changes