Packs up the additions buffer.
This subroutine packs all trains from the addition buffer into a single tensor train, but does not perform the recompression. Normally you should not call it directly, use ttclass/shrink.m instead.
tt - tensor train object with unprocessed additions
ttout - tensor train with additions buffer absorbed into the cores of the tensor, but not recompressed
Additions are expensive in the TT format, and so the tensor train module delays them as much as it can, and only processes them in batches. This function is a part of the processing path.
Version 2.1, authors: Dmitry Savostyanov