Deduplicate group lookup, all group blocks should accept "rand" as an option

[joshlory]

These two methods do basically the same:

dance-party/src/p5.dance.js

Lines 563 to 581 in f0c34bc

	changeMoveEachLR(group, move, dir) {
	group = this.getGroupByName_(group);
	if ((move === "rand") && (group.length>0)) {
	move = this.getNewChangedMove(move, group[0].current_move, false);
	}
	group.forEach(sprite => {
	this.changeMoveLR(sprite, move, dir);
	});
	}
	doMoveEachLR(group, move, dir) {
	group = this.getGroupByName_(group);
	if ((move === "rand") && (group.length>0)) {
	move = this.getNewChangedMove(move, group[0].current_move, true);
	}
	group.forEach(sprite => {
	this.doMoveLR(sprite, move, dir);
	});
	}

So does this method, but it doesn't account for the "rand" case:

dance-party/src/p5.dance.js

Lines 851 to 856 in f0c34bc

	setPropEach(group, property, val) {
	group = this.getGroupByName_(group);
	group.forEach(function (sprite){
	this.setProp(sprite, property, val);
	}, this);
	}