egion strings
--------------

We could consider adding a helper function to ``astropy-regions`` or Gammapy
that we call everywhere in Gammapy at the start of functions or methods that
have a ``region`` argument, like this::

    >>> from regions import DS9Parser
    >>> def make_region(region):
    ...     if isinstance(region, str):
    ...         return DS9Parser(region).shapes[0].to_region()
    ...     else:
    ...         return region
    ...
    >>> make_region("image;circle(42,43,3)")
    <CirclePixelRegion(PixCoord(x=41.0, y=42.0), radius=3.0)>
    >>> make_region("galactic;circle(42,43,3)")
    <CircleSkyRegion(<SkyCoord (Galactic): (l, b) in deg
        (42., 43.)>, radius=3.0 deg)>

This is convenient, a simple ``region="galactic;circle(42,43,3)"`` is much
shorter than having to remember and type this equivalent code::

    >>> from astropy.coordinates import SkyCoord, Angle
    >>> from regions import CircleSkyRegion
    >>> center = SkyCoord(42, 43, unit="deg", frame="galactic")
    >>> radius = Angle(3, "deg")
    >>> region = CircleSkyRegion(center, radius)
    >>> region
    <CircleSkyRegion(<SkyCoord (Galactic): (l, b) in deg
        (42., 43.)>, radius=3.0 deg)>

There is precendence for this pattern in Gammapy, we usually pass ``Quantity``
and ``Angle`` arguments through via e.g. ``angle = Angle(angle)`` in Gammapy
functions, and often call them with strings ``angle="3 deg"``.

This ``make_region`` function could also be the place where we type checks and
do the ``SkyRegion`` to ``PixelRegion`` conversion, avoiding duplicated code for
this in each Gammapy function that has a ``region`` argument, like this::

    from regions import DS9Parser, SkyRegion, PixelRegion

    def make_pixel_region(region, wcs=None):
        if isinstance(region, str):
            region = DS9Parser(region).shapes[0].to_region()

        if isinstance(region, SkyRegion):
            return region.to_pixel(wcs)
        elif isinstance(region, PixelRegion):
            return region
        else:
            raise TypeError("...")

The drawback of adding this now is that everyone will start using it and we
should after Gammapy v1.0 keep supporting it, i.e. we are committing to the DS9
region format in Gammapy, without having considered other possible formats in
detail, or having an equivalent solution in place for ``SkyCoord``.