cosmetics
[fargo] introduce sanity check in shift_int
[si] take selfgravity into account while calculating radial balance
[si] add new trick that may prevent creation of artifacts on outer edge
[si] don't allow dust to gain to much radial velocity on the outskirts of the disk
[si] take selfgravity into account while calculating radial balance
[si] add new trick that may prevent creation of artifacts on outer edge
[si] don't allow dust to gain to much radial velocity on the outskirts of the disk
[si] flatten the profile on the outskirts of the disk, correct dust velocity in low density regions
[si] smooth density only if gradient is positive in at least one cell