Implement loading capabilities of DataManager

[wheelerMT]

Feature Request

Currently, the DataManager classes can only save data, they cannot load it. When working on #19 I found I was having to manually load the data saved by a DataManager using H5py. Instead, a DataManager should have a load_data method which simply returns a h5py.File instance of the data file, which can then be accessed.

Since this will be common to all DataManager instances it should be implemented in the abstract _DataManager class.

Considerations

This should have appropriate error checks which catch if the user has not already specified the filename and data_path attributes.