For compatibility with unported examples, the old typedef is still kept,
but IndexType should be removed from grids eventually.

It was unused and pointless (use entity.getMeasure() instead).

Doing this in a destructor is the only place where we can guarantee that
no open file descriptors will be left behind.

I was running a load method on some object in a loop, which was
returning false most of the time, and eventually the process run out of
file descriptors because the file was not closed properly.

The original structure was not a set in the mathematical sense, but a
map - renamed to reflect this.