Silence warnings from nanmin and nanmax.

When all of `raw_value` consists of NaNs (which seems like a legitimate use-case, given that we already allow some of the values to be NaNs), the `nanmin` and `nanmax` calls in `ImageData.get_bounds` emit a `RuntimeWarning`.

This PR avoids that `RuntimeWarning` by replacing `nanmin` and `nanmax` with `fmin.reduce` and `fmax.reduce` calls.