Add possibility to add fill values on arithmetic operations

[jecisc]

It is possible to do operations such as divide between 2 DataSeries. In the case of a DataSerie containing nils, we consider that the return of the operation should be nil.

	| a b c |
	a := #( 2 4 nil ) asDataSeries.
	b := #( 1 nil 3 ) asDataSeries.
	c := #( 2 nil nil ) asDataSeries.

In some case the user might want to give a fill value instead of using nil. I propose that we add methods like #divide:fillValue: or #add:fillValue: that would use the fillValue in case we have a nil.

fillValue: nil or float value, default nil

Fill existing missing (nil) values, and any new element needed for successful Series alignment, with this value before computation. If data in both corresponding Series locations is missing the result of filling (at that location) will be missing.

[olekscode]

I would rather handle the "sent to nil" exception in those cases. Or use a combination of two methods: fillNil and the arithmetic operation. Otherwise we would need to provide a special method for each operation and it would become ugly... Also fillNil is not the only preprocessing that we might need to do