lib: fix MemoryUsage for positional and component objects

[fingolfin]

Fixes #897

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[frankluebeck]

This looks all good to me (except that in the "generic fallback method" the last else clause could be deleted).

[fingolfin]

@frankluebeck You mean this clause?

I don't mind deleting it, but I had a reason why I left it in: Namely as a reminder that there can be other object types... Though the comment is indeed misleading! For such objects can actually contain subobjects, we just don't have a way to access them! For example, in SingularInterface and NormalizInterface we do this. Perhaps an InforWarning would be appropriate? But then we'd have to take care to only show it for external objects

So, should I remove this? Or instead change it to something like this:

elif TNUM_OBJ_INT(o) >= FIRST_EXTERNAL_TNUM then
  Info( InfoWarning, 1, "MemoryUsage encountered an external object, results may be too low" );

Note that I do not suggest to use TryNextMethod because this is already the fallback method.

[frankluebeck]

Yes, I think the else can vanish. But the proposed warning for new objects with external TNUM sounds like a good idea. For such objects a method for MemoryUsage should be installed.

(Actually, for objects like in the SingularInterface it is not obvious what MemoryUsage should return. Just the space occupied within GASMAN or also the external space, where the latter may not be easy to determine.)

[fingolfin]

Yeah, it's not so clear -- so a warning seems useful, so that one at least knows that there is a hidden iceberg... :)

[fingolfin]

I added the warning, but I had to move MemoryUsage to a new file, as InfoWarning is not available in object.gi. But I actually think this is better anyway.

[fingolfin]

I added a warning that is shown when MemoryUsage is invoked for an external object for which no custom method has been installed. To do that, I had to move MemoryUsage from object.{gi,gd} to new files, as InfoWarning is not available in object.gi. But I actually think this is better anyway.

Also rebased this on master, to resolve a conflict with that.

@frankluebeck Does that seem OK to you now?

[ChrisJefferson]

I'm happy with this.

[fingolfin]

So, merge this? Rebase it? Should I do it myself?

[ChrisJefferson]

Just taking a brief break from merging to allow all tests to pass in master. Then I'll rebase this in too.