Support more event-centric binary operations via sc.lookup

[SimonHeybrock]

Context

See Event-centric arithmetic, in particular multiplication and division. Currently this only supports multiplication and division.

Suggestion

I suggest to extend this for addition and subtraction.

Motivation

For WFM stitching we are currently using an approach where we bin the data, apply a bin-centric operation, then concat. This is actually quite slow, and I think it can be done much simpler using time_of_flight = da.bins.coords['time_offset'].bins + sc.lookup(subframe_offset). This also appears to be faster (on synthetic data).

What needs to be done

1. Add support for + and -. This should be simple.
2. Add support for operating on Variable instead of DataArray. Currently we use the coord to lookup the bin in the lookup table and operate on the data values. In this case we want to use the variable for lookup as well as the operation. This could be done in the Python layer by creating a temporary data array (no copy needed).

2.) is probably optional, since this can be done on the user side.

[SimonHeybrock]

Actually, one can relatively easily achieve the same using sc.lookup to create a temporary. For example, to shift the y-coord:

lut = sc.lookup(myshifts, dim='time')
time = da.bins.coords['time']
time -= lut[time]

With the suggestion from the original message one could:

lut = sc.lookup(myshifts, dim='time')
time = da.bins.coords['time']
time.bins -= lut

In theory the second option (suggested in the initial post) might be a bit faster, but it actually looks less intuitive.

[jl-wynen]

If it turns out that the second option is faster, could the first operation be implemented in terms of the second in appropriate cases?