static_assert on forward iterator requirement in mean rather than silently give the wrong answer

[mborland]

Closes: #1421

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.40%. Comparing base (b430a9f) to head (3718e21).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1427   +/-   ##
========================================
  Coverage    95.40%   95.40%           
========================================
  Files          829      829           
  Lines        69083    69083           
========================================
  Hits         65908    65908           
  Misses        3175     3175           

Files with missing lines	Coverage Δ
...de/boost/math/statistics/univariate_statistics.hpp	99.48% <ø> (ø)

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