Added code to handle constraint violations resulting, in particular,
violations of variable value range restrictions signaled by the
restrict_range trigger function.  This, together with addition of a
PL/pgSQL trigger on insertions and updates to the traits table,
completes task #1958.
Added validation of the value of mean to check that it is within the range of the given variable.
Eliminated calls to to_xs, fixing Redmine issues #1790 and #1851.
Merge branch 'master' of https://github.com/PecanProject/bety