added various dict-like methods to OptionVarDict
added various dict-like methods to OptionVarDict

shrtcww/pymel,AtonLerin/pymel,shrtcww/pymel,juggernate/pymel,leonsooi/pymel,AtonLerin/pymel,assumptionsoup/pymel,david-lassonde-adsk/pymel,cgrebeld/pymel,david-lassonde-adsk/pymel,assumptionsoup/pymel,juggernate/pymel,leonsooi/pymel