Add RuntimeTypeContainer

RuntimeTypeContainer is a new subclass of TypeContainer. The base TypeContainer class can only hold a single object of a given type. When applications handle many objects that share a common base type (i.e., whose complete type is unknown since only a pointer to a base type is held), there was no way to insert these distinct objects into a TypeContainer even if their complete types were distinct.

To resolve this issue, the RuntimeTypeContainer class allows you to insert objects by their base type but use a "declared type-name" as the key. As long as these declared type-names are unique, multiple objects sharing the base type can be held by the container simultaneously.

doc/release/notes/common-runtime-type-container.rst

+Changes to common infrastructure
+--------------------------------
+
+TypeContainer changes
+~~~~~~~~~~~~~~~~~~~~~
+
+A few minor changes were made to :smtk:`smtk::common::TypeContainer`:
++ Methods and variables that were previously private are now protected so that
+  this class can be subclassed.
++ The wrapper class used to store objects in the type container now provides a
+  string token holding the type-name of the inserted type.
+  This is used by the new :smtk:`smtk::common::RuntimeTypeContainer` subclass described below.
++ The ``insert_or_assign()`` method has been renamed ``insertOrAssign()``
+  to be consistent with the rest of SMTK.
+  The original name is deprecated and will be removed in a future version of SMTK.
+
+New RuntimeTypeContainer
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+:smtk:`smtk::common::RuntimeTypeContainer` is a new subclass of TypeContainer.
+The base TypeContainer class can only hold a single object of a given type.
+When applications handle many objects that share a common base type (i.e., whose
+complete type is unknown since only a pointer to a base type is held),
+there was no way to insert these distinct objects into a TypeContainer even if
+their complete types were distinct.
+
+To resolve this issue, the RuntimeTypeContainer class allows you to insert
+objects by their base type but use a "declared type-name" as the key.
+As long as these declared type-names are unique, multiple objects sharing the
+base type can be held by the container simultaneously.
+
+See the class and its test for detailed documentation.