speedup gauss for symmetric rules

[stevengj]

For symmetric rules (corresponding to a HollowSymTridiagonal Jacobi matrix), we can speed up gauss by only doing Newton refinement and eigenvector calculation for half of the points, since the other half are the mirror image.

This gives a 2x speedup for BigFloat gauss rules, where these steps dominate. It doesn't seem to make much difference for Float64 gauss rules, which are probably dominated by the LAPACK eigvals call.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.43%. Comparing base (62eabae) to head (be4e746).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #113      +/-   ##
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- Coverage   98.55%   98.43%   -0.13%     
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  Files           7        7              
  Lines         694      703       +9     
==========================================
+ Hits          684      692       +8     
- Misses         10       11       +1     

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