Add class "IR::model::OpDescription" (#158)

Add class "IR::model::OpDescription" (#158)

* Add class "IR::model::OpDescription"

This commits adds "IR::model::OpDescription" class which used to represent any operation in computation graph

Signed-off-by: Vladimir Plazun <v.plazun@partner.samsung.com>
* Fix minor issues

Add missing newline in CMakeLists.txt
Fix newline after contrib::core::IR::model namespace

Signed-off-by: Vladimir Plazun <v.plazun@partner.samsung.com>
* Fix issues

Remove duplicate check after assert
Add const where applicable

Signed-off-by: Vladimir Plazun <v.plazun@partner.samsung.com>
* Rename nnc_core into core

Update includes structure to evade header collisions
Update CMakeLists.txt

Signed-off-by: Vladimir Plazun <v.plazun@partner.samsung.com>