Per #189, add a sample that points back to the lookup with NearestSmaller
Merge branch 'GH201' into mainline

* GH201:
  Fix the After extension method (#201)
  Added test for After on (non-delayed) Series - shows bug when value does not exists in keys
Fixed erroneous Series.sample implementation
Fix bug whereby Series.diff 1 on empty series is an error
Upgrade to Math.NET Numerics v3.0.0-beta01 #199
Merge remote-tracking branch 'main/mainline' into mainline
Merge remote-tracking branch 'cdrnet/mathnet' into mainline