Add destination sampling window property to Fields

[diptodip]

See Figure 1 of https://opg.optica.org/oe/fulltext.cfm?uri=oe-18-17-18453&id=205150:

As pointed out by @RainerHeintzmann, we could add an origin property (or whatever name) to Field to keep track of a destination sampling window, and then when propagating off axis we can use this method to just update the destination sampling window, allowing off axis propagation without requiring a large field of view.