Reorganize and improve Archive validation

[ChrisKeefe]

• [x] Move validate_checksums() from ProvNode() up to the Parser level (performance and semantics)
• [x] Make checksum validation optional - closes #14
• [x] Add nuance to provenance_is_valid - closes #15
• [x] Prevent missing action.yaml files from blowing up Metadata parsing
• [x] When action.yaml files are missing, set provenance_is_valid to False or equivalent Fail fast on important missing files