Create add and subtract function that behave differently from + and -

Add two functions to la.func: add and subtract. Input is two larrys. Note that lar1 - lar2 returns NaN for the elements that are only in one of the two larrys. But add and subtract will set any missing data to zero (or to some other user specified value?) before doing the addition or subtraction.

We'll need to find the union of all the labels, then morph lar1 and lar2 to the union of the labels, replace NaN with zero if the NaN didn't already exist before the morph, subtract or add.

add and subtract could be convenience functions that call one underlying function that does the work. Input is two larrys and the method ('add', 'subtract').