inconstistent return values for fisher_pdf and exposure_pdf
the fisher_pdf function returns a tuple consisting of pixels and corresponding values and the exposure pdf return an array of length npix with the corresponding values. returning the full array is the more intuitive behaviour. This issue relates to https://git.rwth-aachen.de/astro/astrotools/blob/dff13db83fb3453872befc8303904c4d19b3d726/astrotools/healpytools.py#L256-307