Inconsistency in the example script on restricting to tumor type-specific signatures during refitting

[Hu-JIN]

In the example script https://github.com/parklab/MuSiCal/blob/main/examples/example_refitting.ipynb, this line catalog.restrict_catalog(tumor_type='Skin.Melanoma') gives 15 signatures, not including any MMRD/PPD signatures. However, this is not the case anymore due to 5bfbb16.

As a result, the current behavior is that when is_MMRD and is_PPD are not specified, which are by default True, the obtained signature set will include MMRD/PPD signatures. Therefore, the user has to specifically set both parameters to False to exclude MMRD/PPD signatures. This behavior might not be optimal. A better example/guidance/default/warning message is needed.