Materialx

Updated the MaterialX library to the v1.36 specification. See the [v1.36 changelist](http://www.materialx.org/assets/MaterialX.v1.36.Changelist.pdf) for full details.

Added
- Added support for Element namespaces.
- Added support for NodeDef inheritance.
- Added support for root-level node elements.
- Added support for inheritance attributes on MaterialX\:\:Material and MaterialX\:\:Look.
- Added support for include and exclude attributes on MaterialX\:\:Collection.
- Added the MaterialX\:\:Token class for string substitutions.
- Added the MaterialX\:\:Variant, MaterialX\:\:VariantSet, and MaterialX\:\:VariantAssign classes.
- Added the MaterialX\:\:GeomPath class for geometry name comparisons.
- Added the Collection\:\:matchesGeomString method, for testing matches between collections and geometries.
- Added the Material\:\:getGeometryBindings method, for finding the bindings of a material to specific geometries.

Removed
- Removed the MaterialX\:\:MaterialInherit and MaterialX\:\:LookInherit classes.
- Removed the MaterialX\:\:CollectionAdd and MaterialX\:\:CollectionRemove classes.
- Removed the MaterialX\:\:Override class and support for public names.
- Removed the 'channels' attribute from MaterialX\:\:InterfaceElement.
- Removed the Material::getReferencingMaterialAssigns method (deprecated in Python).

Added
- Added material inheritance support to graph traversal and the high-level Material API.
- Added Material methods getActiveShaderRefs and getActiveOverrides.
- Added PropertySet methods setPropertyValue and getPropertyValue.
- Added Element methods setInheritsFrom, getInheritsFrom, traverseInheritance, hasInheritanceCycle, and getDescendant.
- Added function templates MaterialX\:\:fromValueString and MaterialX\:\:toValueString.
- Added math functionality to the vector and matrix classes.
- Added support for Visual Studio 2017, GCC 7, and Clang 5.

Changed
- Renamed Matrix3x3 to Matrix33 and Matrix4x4 to Matrix44.
- Renamed VectorN\:\:length to VectorN\:\:numElements.
- Updated the PyBind11 library to version 2.2.1.

Added
- Added high-level Material API, including getPrimaryShaderParameters, getPrimaryShaderInputs, getBoundGeomStrings, and getBoundGeomCollections.
- Added methods ValueElement\:\:getBoundValue and ValueElement\:\:getDefaultValue.
- Added support for multi-output nodes.
- Added support for TypeDef members.
- Added StringResolver class, for applying substring modifiers to data values.
- Added example interfaces for the Disney BRDF, Disney BSDF, and alSurface shaders.

Changed
- Renamed method Material\:\:getReferencedShaderDefs to Material\:\:getShaderNodeDefs.
- Renamed method ShaderRef\:\:getReferencedShaderDef to ShaderRef\:\:getNodeDef.
- Renamed method Node\:\:getReferencedNodeDef to Node\:\:getNodeDef.
- Added a 'string' suffix to all accessors for 'node', 'nodedef', and 'collection' strings.
- Combined individual booleans into an XmlReadOptions argument for the XML read functions.

Removed
- Removed method Document\:\:applyStringSubstitutions (deprecated in Python).
- Removed method InterfaceElement\:\:getParameterValueString (deprecated in Python).

Added
- Added support for Python 3.
- Added support for standard TypeDef attributes.
- Added support for values of type 'stringarray'.
- Added method Element\:\:setName.
- Extended Python bindings for Document, NodeGraph, MaterialAssign, and Collection.

Changed
- Modified NodeGraph\:\:topologicalSort to return elements in a more intuitive top-down order, with upstream elements preceding downstream elements.
- Removed special cases for string return values in MaterialX Python, with all strings now returned as 'unicode' in Python 2 and 'str' in Python 3.
- Updated OSL reference implementations.

Fixed
- Fixed handling of empty names in Element\:\:addChildOfCategory.
- Fixed an edge case in Document\:\:upgradeVersion.

Added
- Added OSL source files for the standard nodes.
- Added example document 'PostShaderComposite.mtlx'.
- Added method MaterialX\:\:prependXInclude.

Changed
- Argument 'writeXIncludes' defaults to true in MaterialX\:\:writeToXmlStream and MaterialX\:\:writeToXmlString.

Fixed
- Fixed handling of BindInput elements with missing connections.

Added
- Added a 'viewercollection' attribute to MaterialX\:\:Visibility.
- Added Python support for visibility and source URI methods.

Changed
- Changed naming convention from 'ColorSpace' to 'ColorManagement' in Document methods.
- Split library document 'mx_stdlib.mtlx' into 'mx_stdlib_defs.mtlx' and 'mx_stdlib_osl_impl.mtlx'.