handle registration of scan fields larger than or outside of stack

[paulgfahey]

To allow for automatic transmission of manually labeled retinotopic area masks into the stack, it is necessary to register those fields into the stack.
In some cases, the scans used to generate those areas are much larger than the stack, spanning the whole window. Currently, one of the registration methods crashes if the field is larger than the stack.
In other cases, one or more of those fields may have no overlap with the stack. Desirable behavior in that case may be to avoid attempting registration for fields that are predicted to be outside the stack due to known relative position to other in-stack fields. Output may be to either report a stack location by preserving that relative location, or to output a null location to emphasize that the stack can't inform the location of that field directly.

[ecobost]

We could take the center of the field rather than the entire field, main issue is that the non-rigid registration will have to extrapolate for outside stuff (maybe we could fill it with nans)