use abstract interface with hidden implementation

move GPU constants initialization from constructor to compute method

The 'sysctl' syscall has been strongly deprecated on Linux for ages.
Currently, on old architectures it will spam syslog whenever used, and on
newer ones it's missing from the headers altogether.  Opencv has migrated
away on Linux already, but #includes were left lingering.  This commit
removes them on non-__APPLE__, unbreaking x32 (and probably others).

it is marked as obsolete

Signed-off-by: Yan Wang <yan.wang@linux.intel.com>

- added test for loading legacy files
- added version to new written models
- fixed loading of several fields in some models
- added generation of new fields from old data

used BufferPool mechanism instead