This commit

+ adds methods to `smtk::model::Instance` to generate its tessellation on
  demand from properties defined on it in the model manager.
+ provides a way to specify instance placements as properties on the instance;
  both tabular point coordinates and random placement generation are supported.
  In the future, we envision `smtk::common::Extension`s (see below) providing
  extensible ways to specify instance placements that are application-specific.
+ provides a way to specify that instance placements should be snapped to
  points on model-entity tessellations (including cells and auxiliary geometry).
+ adds `smtk::common::Extension`, a class to allow methods in the smtkCore
  library to invoke methods from other libraries (with additional dependencies)
  if they are present; immediate subclasses should be abstract and implemented
  by concrete children in other libraries.
    + adds a point-locator extension to SMTK and implements it in a new ParaView
      server-side plugin; this is used to snap instance placements to points in
      tessellations of other entities.
    + adds an auxiliary-geometry extension, `vtkAuxiliaryGeometryExtension`, to
      SMTK and implements it in the vtkSMTKSourceExt library; this is used to
      verify that URLs are valid during aux. geom construction, to fetch bounding
      box information of auxiliary geometry, and to allow the point-locator
      extension to perform snapping when loading SMTK files (before auxiliary
      geometry has a corresponding VTK dataset in the CMB application).
+ adds a new operator: `Model Entities - Create Instances` that provides a way
  for users to create an instance of a model entity or of auxiliary geometry;
  each instance may have many individual _placements_ (i.e., locations in space
  where the prototype entity is repeated).
+ adds transform properties to the `Model - Add Auxiliary Geometry` operator so
  that a rigid-body transform can be applied to data as it is read.
+ adds support for deleting Instances to the polygon-session
  `Model Entities - Delete` operator.
+ adds a new operator: `Model Entities - Group Auxiliary Geometry` operator to
  allow creation of hierarchical auxiliary geometry. These new, hierarchical
  auxiliary geometry instances may be instanced, allowing for complex scene
  construction.
+ adds a new templated method to `smtk::model::EntityRef` named `instances()`
  that you can use to ask any entity for a list of places it is duplicated
  in the model.
+ adds output ports to `vtkModelMultiBlockSource` that hold
    + a second multiblock dataset that holds references to each entity serving
      as a prototype to an instance; and
    + a polydata containing placement points for all instances in its vtkPoints.
      Instance tessellation points are assumed to be the origin of glyphs
      that reference blocks in the first output port (the multiblock of
      model and non-image auxiliary geometry).
  These two output ports can serve as inputs to a `vtkGlyph3DMapper` for rendering
  instances. Eventually, the polydata will be replaced with a multiblock of
  polydata point placements, one block for each instance entity (in order to
  allow block-selection in VTK/PV to translate into entity-selection in SMTK/CMB).
+ adds the ability to specify a transform when constructing auxiliary geometry.
  Note that the order of operations has not been thoroughly tested to be consistent
  with VTK, but that is the intent.
+ tracks all VTK-SMTK model/mesh-source objects as they are constructed/destroyed
  in order to make point-location and snapping possible in CMB.

This commit does not:

+ provide ways to change the orientation of instance placements individually
+ provide a way to specify coloring of instances (per-placement or per-instance)

Future merge requests are planned to implement these features.

This commit also improves documentation for model operators by describing
conventions for operator specifications (inputs) and results (outputs).