* Add NodeInput and NodeOutput classes

* Deprecate Node::get_inputs, Node::get_outputs, Node::get_output_inputs. Remove Node::get_input_from and Node::get_output_from

* Privatize most fields of Node

* Make deprecation of descriptor-munching classes optional

* Review comments

* Adapt ReshapeSinking to use raw pointers for NodeInput

* Fix ZDTE (thought I had already done in this branch, weird); style

* wip

* Change get_node_outputs() and get_node_inputs() to return vectors

* Updates after merge

* Whoops, forgot to define these functions

* {NodeInput,NodeOutput} -> {Input,Output}

* Kill shared_ptr in Output

* Move Input and Output into node.hpp

* Templatize the underlying node (sub)type in Input and Output

* Eliminate some get_input_* and get_output_* functions

* Change get_outputs and get_inputs back to their original names; rename NGRAPH_DEPRECATE_IO_DESCRIPTORS to NGRAPH_DEPRECATE_OLD_NODE_APIS

* Miscellaneous cleanup

* More cleanup

* Unbreak CPU build

* Simplify unit tests

* Make Node less friendly

* Deprecate more get_output_* and get_input_* functions

* A couple of PR comments

* Make the deprecation stuff more generally available

* Better comment

* Be more consistent about [] vs. at