[IMP-dev] ObjectContainer disabling checks

I had missed wrapping one of the checks in ObjectContainer with NDEBUG which made adding large numbers of particles painfully slow.

I can't submit a patch since it would conflict with the not yet committed changes. It is simple, the single object insert needs to have its check wrapped with IMP_BEGIN_CHECK(EXPENSIVE); IMP_END_CHECK

And the multipartcle check which is currently CHEAP should be made EXPENSIVE.