New datasets framework

[orperel]

Datasets have been reformatted in this MR to support easy adding of new datasets.

• All datasets should subclass WispDataset, which returns a Batch instance from __get_item__
• Underneath WispDataset, abstract MultiviewDataset and SDFDataset define more specific interfaces.
• NeRFSyntheticDataset and RTMVDataset are new concrete classes, which return a MultiviewBatch.
• Previous SDFDataset separated to concrete classes MeshSampledSDFDataset and OctreeSampledSDFDataset, depending on whether samples are generated directly from mesh or limited by the octree narrowband. Both return a SDFBatch.
• load_multiview_dataset is a convenience method that looks at a given path and tries to determine the correct MultiviewDataset subclass to create (out of all available dataset subclasses). This relies on the optional is_root_of_dataset() @classmethod each dataset may implement to distinguish itself from other formats.

Signed-off-by: operel [email protected]