.opAssign disabled without @disable

[denizzzka via Digitalmars-d-learn]

As Alex Parrill says, on problem was in const member. But this is 
one of the problems, and after fix here still was an error.

But alphaglosined found another problem! For some unknown reason 
here it is need to specify an empty postblit constructor.

Full patch:
https://github.com/denizzzka/r-tree/commit/ca9231df5a8a227aa9a8010b849e2e92a134f8a1

So, my problem is solved. But nevertheless maybe here is a 
problem in the compiler too.