Fix access to Warren_2020 models on Zenodo.

[sybenzvi]

The model_registry_jan22 branch now supports downloading the Warren_2020 models from Zenodo, but we discovered that the untarred HDF5 files available on Zenodo do not cover all of the simulations from Warren et al. As a result, the Warren_2020 unit tests have been temporarily disabled.

It does appear that the three uploaded tarfiles stir_multimessenger_a1.23.tar, stir_multimessenger_a1.25.tar, and stir_multimessenger_a1.27.tar do contain the missing files. So we need to:

• Update the Warren_2020 constructors in snewpy.models.ccsn and snewpy.models.loaders to use the tar files rather than the individual HDF5 files.
• Reactivate the unit tests.

[JostMigenda]

For now, in #184 we have returned to downloading the Warren_2020 model files (individual .h5 files, not the .tar files) from GitHub. See discussion in there for some more problems we’ve encountered with unit tests and downloading tar files.