Filter jbrowse dataset using constant JBROWSE_TOOL_ID (get_datasets)

The following discussion from !45 should be addressed:

• @lgueguen started a discussion:

  Favor the filters inside the bioblend get_datasets method. Use tool_id instead of name an extension (more reliable):

  jbrowse_datasets = self.instance.datasets.get_datasets(tool_id=constants_phaeo.JBROWSE_TOOL_ID)