Add a function to convert WCS-based DS9 regions to supported coordinate frames

[larrybradley]

Region objects represent abstract shapes that are not tied to any particular image or WCS transformation. Therefore, WCS-based DS9 coordinate frames are not supported, e.g., physical, detector, wcs[0;a-z], etc.

As a workaround, one can load such region files into DS9 with their corresponding image/WCS and then save the region with a coordinate frame supported by this package (e.g., image, ICRS, FK5, etc.). We could explore adding a function that does this conversion in the regions package.

ref: #403