* improve thresholding tutorial, fix grammar issues and incorrections

* keep full list of simple thresholding types

* Mac support for op inference engine

Adding condition to check for mac and add corresponding libraries

* Adding mac support in test cases

* Postpone variable definitions

* dnn: reduce scope of 'Mat image' variable

- models are moved out from OpenVINO package
- need to download them separatelly

backporting of commit: f2803aba

- use relative paths for includes
- fix PCH generation with GCC (dummy file has been used)

backporting of commit: 45428b89

- exceptions from constructor will not cause destructor calls

Seeking outside of the current block doesn't update actual data,
so getByte()/etc will get wrong data from stalled buffer.

2Gb+ images can't be handled properly by current implementation

backporting of commit: 50a6f9d2

required for close calls.

- don't share .cpp files (PCH support is broken)

backporting of commit: 8afb2f0c

* Fix precision in tests for MyriadX

* Fix ONNX tests

* Add output range in ONNX tests

* Skip tests on Myriad OpenVINO 2018R5

* Add detect MyriadX

* Add detect MyriadX on OpenVINO R5

* Skip tests on Myriad next version of OpenVINO

* dnn(ie): VPU type from environment variable

* dnn(test): validate VPU type

* dnn(test): update DLIE test skip conditions