Fixed OpPathPreview bugs.

- Errors if the op result type had no parameter mapping. This was from using an OpHolder instead of a ParameterisedHolderNode - since we don't use the computation facilities of the node and instead run the op ourselves, we don't need to use an OpHolder.
- Using the default IECore ClassLoader and ignoring any custom ones. This was because we were using OpHolder.setOp( className ) rather than loading the op with self.getPath().load().