`cube.output_first_header` is missing several comments

[altuson]

cube.output_first_header is missing comments for several keywords. One way to fix this is to update line 39 of fits.py. Instead of

header_dict = {
        k: fits.Card(
            k,
            hdu.data[k][0]
            if isinstance(hdu.data[k][0], str)
            else np.nanmedian(hdu.data[k]),
            c,
        )
        if (k in hdu.data.columns.names)
        else fits.Card(k, hdr[k], c)
        for k, c in zip(hdr.keys(), hdr.comments)
        if (
            (k not in delete_kwds)
            and (not k.startswith("TTYPE") and (not k.startswith("TFORM")))
        )
    }

use this instead

header_dict = {
        k: fits.Card(
            k,
            hdu.data[k][0]
            if isinstance(hdu.data[k][0], str)
            else np.nanmedian(hdu.data[k]),
            c if c != "" else hdr[k],
        )
        if (k in hdu.data.columns.names)
        else fits.Card(k, hdr[k], c)
        for k, c in zip(hdr.keys(), hdr.comments)
        if (
            (k not in delete_kwds)
            and (not k.startswith("TTYPE") and (not k.startswith("TFORM")))
        )
    }

[Nschanche]

I've implemented this with PR #22.