Update build instructions.
Fix initialization issues in CGM.

Initialize CGM only once, so that opening and closing
multiple bridge sessions is not a problem.
Merge topic 'fix-cgm-session'

d6a89a4 Fix initialization issues in CGM.
f12d565 Update build instructions.
Estimate surface normals for surface tessellations.

Since CGM does not provide normals or (u,v) coordinates
for its tessellations of faces, we run things through
vtkPolyDataNormals. But this is tremendously slow.
Try reusing vtkPolyDataNormals instance for speed.

... unfortunately, this does not appear to improve the situation.
However, loading `t6dcc.igs` does eventually lead to an exception
being thrown (Standard_ConstructionError).
Break userguide model doc up; add property section.
No need to copy figures anymore with Sphinx.

Also, remove ninja-doc from the default build.
Be aware that this can cause "make install" to
miss installing the documentation, so packaging
must "make doc-userguide; make install".
Allow per-model and per-entity normal generation.

Look for the "generate normals" property on models and
tessellated entities to see if behavior is specified.
This also adds a flag to the multiblock model source
that forces normal generation off by default, since it
can be very slow.
Fix a python test.
Merge topic 'polynormals'

4dded31 Fix a python test.
0859168 Allow per-model and per-entity normal generation.
a9a617d No need to copy figures anymore with Sphinx.
0129e29 Break userguide model doc up; add property section.
f212fbc Try reusing vtkPolyDataNormals instance for speed.
8ce9a70 Estimate surface normals for surface tessellations.