Update FrameCorrector to save XY translation

[sumiya-kuroda]

Now, the FrameCorrector object also saves the amount of xy translation. It also supports the lazy_data_loader() class, and you can access the translation information by

motion_correction_dataloader = registrationarrays.RegistrationArray(registration_obj = your_frame_corrector, data_to_register = your_data)
motion_corrected_data_slice, xy_translation = motion_correction_dataloader[time_slice]

This is useful when you want to save data in NWB format with pynwb.ophys.MotionCorrection class.

[apasarkar]

Awesome @sumiya-kuroda - thanks for the contribution! It makes sense to me that frame_corrector should output the registered data + the shifts (for both piecewise rigid and rigid registration). Looks like the changes you've made do that.

However, for the RegistrationArray, it's very important that it behaves exactly like an array; i.e. my_reg_array[indices] should output an array. I've included some suggestions on what might need to be done to work around that.