BUG/API : relax validation in hist

Relax the validation on input data to hist.

 - up-convert scalar to length 1 list
 - don't error out for length 1 input data

closes #3882
TST : added test of relaxed validation in hist
Merge pull request #3883 from tacaswell/hist_validation_relax

BUG/API : relax validation in hist