Add overlap/buffer handling to core processing

[JRM]

Hi,

I would like to suggest extending the overlap mechanism integrated in split_in_chunks_dialog.py and integrating it into the core of Photoscan.

This feature is critical to get perfectly joined chunks without artifacts but it only works one way at the time of the creation of the chunk by the script, there is no way* to reduce the chunk to its original size before merging it back with the others ( * without using python).

LIDAR chaintools like LASTools or PDAL include the concept of buffers when tiling a dataset, it allows to work on each chunk with overlapping data and discard the flagged buffer when needed.

I think there should be three functions :
- add overlap (both a parameter and a processing stage)
- use overlap
- discard overlap

These functions would make it easier to have a full photoscan workflow without painfully exporting/importing on large projects.

regards,
JR