Ensure that all quadrature rules store their nodes in sorted order.

[JSorngard]

This PR adresses #102.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.06%. Comparing base (9566364) to head (350e362).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
+ Coverage   94.94%   95.06%   +0.11%     
==========================================
  Files          10       10              
  Lines        1939     1986      +47     
==========================================
+ Hits         1841     1888      +47     
  Misses         98       98              

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[JSorngard]

Sorting reduced the accuracy of the integration in the test of

GaussHermite::new(5).unwrap().integrate(|_| 1.0)

I reduced the required threshold of the test to 1e-14 from 1e-15, but I think it is worth flagging in the discussion.

[JSorngard]

@DomiDre what do you think of this?