Fix metadata fill value for int dtypes #933
Description
Reviewing the tests/docs for release and doing some work I noticed a warning coming from the new metadata code. Its harmless when we're dense, but its not too hard to clean it up, patch coming soon.
/Users/gbwright/work/ASPIRE-Python.fsc/src/aspire/source/image.py:528: RuntimeWarning: invalid value encountered in cast
self._metadata[metadata_field] = np.full(self.n, fill_value).astype(