Re-factored the whole external potential mechanism to match what is done for cooling

Also, renamed 'disk' to 'disc' and improved the documentation and description of the parameters.

@pdraper this is just code rearrangement, no changes to the actual physics has been done.

Compiles OK, but I cannot run the ExternalPointMass example. That crashes when queueing an init task that has no super cell. Probably an error from !226 (merged), related to #215 (closed)?

Yes, that is how I noticed the issue #215 (closed). The master is currently broken in gravity-only mode and so is this branch.

Sorry, should have flagged this in the text.

Added 1 commit:

• 910224ba - Need new PointMassPotential section

Added 1 commit:

• 4a86f7b7 - Remove bad and unused PointMass section

OK, seems to work except for this issue. I'll accept for now.

Status changed to merged

mentioned in commit b65b49f4