Added ieMeshInterpolation support for Houdini.

ToHoudiniPolygonsConverter will create a primitive attribute named "ieMeshInterpolation" and set it according to the MeshPrimitive::interpolation. "catmullClark" maps to "subdiv" and "linear" maps to "poly". FromHoudiniPolygonsConverter will detect the "ieMeshInterpolation" attribute and set the interpolation accordingly. The attribute name and values match IECoreMaya functionality.
Merge pull request #8 from andrewkaufman/houdiniMeshType

Added ieMeshInterpolation support for Houdini.
Adding a new way to convert in CachedConverter where we pass a custom converter object that also is responsible for giving a unique hash.
Adding new constructor to IECoreGL::MeshPrimitive where we can pass the original non-triangulated topology and then, addPrimitiveVariable will triangulate any primvars added. This will be useful for updating scenes in the future.
Also using the CachedConverter for all triangulations and normal computation.
Using the new maxVerticesPerFace() from Mesh when converting to a GL mesh.
Allowing .lscc files to be loaded
IECoreMaya fixed dag menu GL bug for sceneShapes
Fixing SceneCacheNode function signatures

Making scene() static since it doesn't use any member data or node parameters. Making get*() const.
renaiming "build" to "expand" and "clean" to "collapse" to match IECoreMaya naming
Separated Expand and Collapse functionality.

SceneCache OBJs now expand and collapse from separate functions (and buttons). The Expanded toggle marks the current state, and may be queried by external processes (such as a ROP) to determine if the user has fiddled with the underlying scene.
Changed behaviour of SubNetwork expansion to supress child objects.

This is necessary to ensure that objects are not expanded prior to children of the same level, providing a coherent definition for the expanded/collapsed state of each node.
Allowing custom attribute handlers to be registered with HoudiniScene
OBJ_SceneCacheTransform registers the LinkedScene attribute with HoudiniScene to represent unexpanded transforms as links.
LinkedScenes can now be written with the SceneCache ROP.

The ROP will now write attributes, the primary use of which is the link attribute for lscc files. Required wrapping the live HoudiniScene with a LinkedScene when writing non-linked files, to ensure full expansion of links.
Couple of defines for building against PRMan17.
Merge pull request #11 from valeriebernard/sceneShapeGLIssue

IECoreMaya fixed dag menu GL bug for sceneShapes
Merge pull request #12 from andrewkaufman/houdiniLinkedScenes

LinkedScenes in Houdini
Reorganized parameter creation for OBJ_SceneCache nodes to avoid horrible munging of PRM_Templates.
Added tagFilter to OBJ_SceneCacheTransform

The filter is used to control visibility when expanding nodes. Available tags are presented in the drop down menu.
Allowing custom tag handlers to be registered with HoudiniScene.

Note that this does not match exactly with IECoreMaya. This is a vector of tag handlers, rather than registered by specific tag name. This allows one handler to provide a list of tags all at once, which seems more in line with the
intention of SceneInterface::readTags().