Overwrite array values of base data sparsely

[RFEphemeration]

So that the base data can be

@character_template : @character_definition { xp_required : [100, 200, 300, 400, 500, 600] }

and the override can be something like

character_example : @character_template { xp_required: [ 4 : 550 ] }

though the syntax has yet to be decided