Update v1.3.0; refactoring for array-based datasets
Largely rewrote datasets module interface classes to support array-based packets processing (e.g. for cosegmentation and registration), & added new utility/helper functions to the module.
Besides:
- DataPrecacher now relies on an atomic flag to avoid re-requests for almost-ready packets.
- Added VAP trimodal (2016) dataset implementation specialization header.
- Fixed compilation error for GCC 6.2 in BSDS500 dataset implementation.