v0.20.0

Singular v0.20.0

Diff since v0.19.0

Merged pull requests:

• rename orderings: revlex->invlex,rp->ip...introduce deginvlex (#722) (@hannes14)
• vdim: avoid overflow (#730) (@hannes14)
• Bump Aqua to 0.8 and use more defaults (#731) (@lgoettgens)
• Version 0.20.0 (#733) (@fingolfin)