BF: Fix type overflow on images with fixed-width integer type

Paul McCarthyrequested to merge
paulmc/fsleyes:bf/type-overflow into main

The fsl.data.image.Image.dataRange property returns image minimum/maximum values in the datatype of the image (e.g. np.int16). This could cause overflow issues when calcaluting the image range. All uses of the Image.dataRange have been updated so that the values are coerced to floating point before any further processing.

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