* Split op skeleton

* Two ways to construct a fused Split to be able to use it in onnx importer

* refactor: move the util::split() helper functions to the core

* Split's decompose_op() implementation using a helper function

* Use fused Split in the onnx_importer

* Code formatting

* PR feedback

* Split helpers moved to ngraph/builder

* Basic UT - split a 1D tensor to 3 equal parts

* UT: Split 2D tensor into variable length parts

* Code formatting

* Catch the proper type of exception in the onnx_importer split()

* Initialize members in the correct order

* Type prop tests for Split

* Code formatting

* PR feedback