Make get_map return healpix map

[SergeiDBykov]

Hi!
I think it would be very handy and cool to have a healpix map returned when one uses plot_map function from 'data' module. Also the hires data should be included if needed.

it requires a modification only of a few lines in core.py, as below

    @classmethod
    def plot_map(cls, map_name, plotname=None, hires=False):
        """
        Full-sky plot (mollweide projection) of the map.

        **Note:** this method needs ``healpy`` and ``matplotlib``.

        Parameters
        ----------
        map_name : ``str``
            Name of the map to be plotted. Use ``show_maps`` method
            to see a list of all available maps.
        plotname : ``str`` or ``None``, optional
            Name of the file where the plot will be saved. If ``None``,
            the plot is shown but not saved. Defaults to ``None``.
        hires : bool
            whether to use high resolution map
        Returns
        -------
        plot : ``numpy.ndarray``
            2D numpy array with the plot.
        hpmap : ``numpy.ndarray``
            1D numpy array with the healpix map being plot.
        """
        import healpy as hp
        import matplotlib.pyplot as plt

        cls._check_map(map_name)

        if not hires:
            hpmapfile = "{}_{}_healpix_lowres.fits".format(
                cls._map_type, map_name)
        else:
            hpmapfile = "{}_{}_healpix_hires.fits".format(
                cls._map_type, map_name)

        hpmapfile = os.path.join(cls._data_path, hpmapfile)
        hpmap = hp.read_map(hpmapfile)

        title = "{} ({})".format(map_name, cls._maps[map_name])
        if cls._map_type == "h1_nh":
            unit_label = "cm-2"
            minval, maxval = 1e19, 3e22
        else:
            unit_label = "mag"
            minval, maxval = 0, hpmap.max()

        plot = hp.mollview(
            hpmap,
            title=title,
            norm="hist",
            min=minval,
            max=maxval,
            unit=unit_label,
            return_projected_map=True,
        )

        if plotname is None:
            plt.show()
        else:
            plt.savefig(plotname)

        return plot, hpmap

And for this reason, returning low/hi resolution map should be an argument.
Healpix NH maps are useful for creating masks for, say, X-ray surveys.

Thanks!