Dp point cloud data type, and a separate .py file for datatype

[zijiewu3]

• Adding a new data type of DP_POINT_CLOUD for point clouds containing images (e.g. diffraction patterns) instead of spectra. Most of the functionalities are similar to the original POINT_CLOUD. Visualizer is extended to support DP_POINT_CLOUD too.
• A change that should not but can potentially affect other functionalities, if I didn't manage to catch everything I need to change: I moved the DataType class out of dataset.py into a new file so that it can be accessed by other modules (specifically dataset_viz.py) without causing cyclical import. It also makes a bit more sense to me that DataType is defined as a standalone enum equally accessible to all modules, instead of as a child class of dataset.py.