#1139 broke ProtoPlanetaryDisk

[LourensVeen]

Describe the bug
#1139 tried to rename an argument to amuse.ext.protodisk.ProtoPlanetaryDisk.__init__, but the logic is wrong and now examples/textbook/solar_cluster_in_semilive_galaxy.py is broken.

It calls ProtoPlanetaryDisk with Rmin set to a value that isn't 1, which causes a ValueError to be raised because Rmin isn't the same as the default value of radius_min.

The intent of #1139 was to make it so that you could set either Rmin or radius_min or both if they were consistent, but that's not what the code currently does.

[LourensVeen]

Oh, and on lines 95 and 96 we refer to Rmin and Rmax, which may not be set. That should be self.Rmin and self.Rmax.

And while we're at it, the change left us with radius_min and radius_max parameters for __init__, but attributes still called Rmin and Rmax, so now it's inconsistent. Better to have attributes radius_min and radius_max with Rmin and Rmax properties referencing them for backwards compatibility.

I did a poor job reviewing this.