Implementing Class and Datatype discovery

The following changes have been made:
* adding necessary types including datatype and class descriptor types and types allowing constraints
* for struct datatypes we now have a get_type_descriptor used by the Class Manager
* add the intermediate NcManager entitiy and updated NcDeviceManager to be based on it instead of directly from NcObject
* all classes based on NcObject now offer a get_class_descriptor used by the Class Manager
* add a new Class Manager entity used for datatype and class descriptor discovery via properties or methods
* instantiate the new class manager in main inside the root block
* the root block no longer offers itself in the results of find_members_by_class_id
* added better validation for message handles which now send an Error message when invalid
* tweak various edge cases so they respond with more appropriate statuses

Author: cristian-recoseanu

README.md

@@ -28,14 +28,14 @@ The following features are working:
     * Implementing the generic Set method of any object to set the value of any property ([NcObject](https://specs.amwa.tv/ms-05-02/branches/v1.0.x/docs/Framework.html#ncobject))
 * Offering an NcBlock implementation and advertising a root block and nested block
     * Implementing all [NcBlock](https://specs.amwa.tv/ms-05-02/branches/v1.0.x/docs/Framework.html#ncblock) methods
+* Offering a [Class Manager](https://specs.amwa.tv/ms-05-02/branches/v1.0.x/docs/Framework.html#ncclassmanager) implementation
+    * Implementing [class discovery](https://specs.amwa.tv/ms-05-02/branches/v1.0.x/docs/Managers.html#class-manager)
+    * Implementing [datatype discovery](https://specs.amwa.tv/ms-05-02/branches/v1.0.x/docs/Managers.html#class-manager)
 
 ## To do
 
 The following features are planned:
 
-* Implementing the [ClassManager](https://specs.amwa.tv/ms-05-02/branches/v1.0.x/docs/Framework.html#ncclassmanager)
-    * Implementing [class discovery](https://specs.amwa.tv/ms-05-02/branches/v1.0.x/docs/Managers.html#class-manager)
-    * Implementing [datatype discovery](https://specs.amwa.tv/ms-05-02/branches/v1.0.x/docs/Managers.html#class-manager)
 * Implementing the [IS-04 registration workflow](https://specs.amwa.tv/is-04/releases/v1.3.3/APIs/RegistrationAPI.html) so we can register resources in an NMOS registry and maintain the registrations via heartbeats
 * Implementing the [NcReceiverMonitor](https://specs.amwa.tv/nmos-control-feature-sets/branches/main/monitoring/#ncreceivermonitor) model
 * Implementing the [NcSenderMonitor](https://specs.amwa.tv/nmos-control-feature-sets/branches/main/monitoring/#ncsendermonitor) model