Update metadata parser/transformer names to reflect metadata format instead of data processing level

[ngachung]

To support #123 it makes sense to reuse the metadata parser for SNPP L0, so it makes sense to refactor the metadata parser to

1. S4PA metadata - for SNPP L0 and CHIRP L1
2. CAS metadata - for SNPP L1

Remove references to L0 vs L1 in code to avoid confusion in the future