MAVEN IUVS dictionary can't be converted to tplot variables

[jameswilburlewis]

See test_load_kp_iuvs_data in the MAVEN test suite.

The dictionaries from reading the IUVS data files can't be converted with maven_kp_to_tplot(). Maybe there needs to be another conversion routine for that type of data? For now, when loading both kp insitu and iuvs data, it only returns the insitu variables and discards the iuvs dictionary (because downstream routines are expecting a list of variables, not a dicitonary).