Patch for loading multiple ieCoreHoudini plugins.

Houdini loads all plugins on the path, regardless of whether they have been seen before. This means that when installing things in a studio setup, several versions of the plugin may be loaded, for example a user install for testing and the central install. This commit prevents such situations from seg faulting, and instead prints a warning to stderr. Currently the only conflict will be the definition of GU_CortexPrimitive. Since the correct library will still be loaded, I think the warning can be ignored in most situations.
Fixed IECoreAlembic to use GeometricTypedData appropriately.

Previously we were returning TypedData<vector<V3f>> which doesn't include the geometric interpretation, and isn't expected by much of the rest of Cortex. Now we use the appropriate GeometricTypedData type and correctly specify the geometric interpretation based on the information in the Alembic file.

Fixes #28.
Fixed IECoreRI test which explicitly specified a display with +.

It was working around the lack of support for multiple displays in IECoreRI::Renderer, and since that was fixed had been erroring.
Merge pull request #114 from andrewkaufman/houdiniPluginPatch

Patch for loading multiple ieCoreHoudini plugins.
Merge pull request #116 from johnhaddon/alembicFix

Fixed IECoreAlembic to use GeometricTypedData appropriately.