BUG: Fix bug in which adding memory particles to a KDTree hash also adds them to the output of link_iter().

Added assert which may catch this if _verify_integrity is turned on.
Fixed placement of comment around memory code in link_iter()
BUG: Fix undefined frame_no in batch()
Merge pull request #58 from nkeim/fix-parallel

BUG: Fix undefined frame_no in batch()
Merge pull request #55 from nkeim/fix-copybug

Memory/KDTree bugfix (second try)