Add option to remove the first acquisition row in IMC images.

[nathansteenbuck]

Due to the laser, we often observe higher intensities in the first row (X = 1) of IMC images.
I would suggest to add an argument for the preprocess functionality to remove the first row from the image (e.g. rm_first or remove_first). By default it should be set as False.

[jwindhager]

Note: IIRC this happens because, for the first row, the laser does not just ablate the "stage delta", but the full spot size.