Scripts: Make ScoreAccessor utility available for other script engines.

With the removal of setNextScore in #6864, script engines must use
the Scorer to find the score of a document.  The DocLookup is updated
appropriately to do this, but most script engines require a Number to be
bound for numeric variables.  Groovy already had an encapsulation for
this funtionality, and this moves it out to be shared with other script
engines.

closes #6898
Scripting: Fix use of _score with mvel after removal of setNextScore.

mvel does not play nicely with Number, so replacing the score with
a boxed Float on every document is still necessary.