Additional Support of Datasets input

DL3 Data component provides the general overview of the inputs used in asgardpy for the data reduction of DL3 data to DL4 data, following the Gammapy documentation on Data Reduction (DL3 to DL4)

Asgardpy can also read from already reduced DL4 datasets, like the examples from gammapy-data, using the module io_dl4. This is done by using input_dl4 = True or input_dl4 = True and filling dl4_dataset_info and dl4_dataset_info respectively.

This can be done for an additional compilation of DL4 datasets to run the joint likelihood analysis using Gammapy. Tests are included for checking an example of this support.