* onnx: add 'constant' operator
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: getting attribute value by name
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: fix code style
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: fix clang compilation warnings
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: exception
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: add 'split' operator
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: add public interface
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: add initial unit test for importer
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: initial implementetion of operator' set
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* [WIP] Unit test for split operation.

* Fix Split Op bounds calculation + UT

* clang format

* Split Op with variable parts unit test.

* Remove unused headers

* Add missing ngraph install prefix for cmake in travis Dockerfile.

* Remove -Wno-zero-as-null-pointer-constant

* Code review

* Apply clang-format-3.9

* Add missing onnx_import interface files to CMakeList

* Copyright notice format