Add mixed barcode rescue to SpotDecoding

[elaubsch]

This PR adds a function _rescue_mixed_spots to the SpotDecoding application. This function addresses the case of mixed barcodes caused by spatial crowding of spots. An argument rescue_mixed has been added to the predict method of SpotDecoding to toggle this function. A test case has been added to cover this function. Print statements have been added to make the prediction more verbose to make the amount of error correction more obvious.

The function _rescue_spots has been refactored to _rescue_errors, because there are now two methods for rescuing spots. The exposed argument rescue_spots has also been changed to rescue_errors.

Regardless of error correction, two items have been added to the dictionary returned by SpotDetection.predict.

1. spot_index indexes the spots, because rescue_mixed_spots introduces the case that two gene assignments can be made for the same spot. In that case, a new entry is added to the output with the same index as the original spot.

2. source details the origin of a prediction. Its values can be:

   • 'prediction' from SpotDetection.predict
   • 'error rescue' from rescue_errors
   • 'mixed rescue' from _rescue_mixed_spots