Fix nodata values at source edges

This is in case anyone else runs into the same issue, really.

I was having issues with artefacts at the edges of source images, caused by nodata values. Adding a call to numpy's `nan_to_num` in data_to_rgb (encoders.py) fixes it:

data /= interval
data = np.nan_to_num(data, True, 100000.0)
data = np.around(data / 2**round_digits) * 2**round_digits

(100000.0 is from the -b and -i values of my invocation and shouldn't be hardcoded, obviously.)

Start by reading data_to_rgb in encoders.py and trace how the -b and -i values determine the interval and nodata handling. Verify the edge case with source images containing nodata values, and ensure the resulting encoded output has no edge artefacts without hardcoding invocation-specific values.

Written by the indexing model from the issue text.