Refactored Snakemake workflow and generate example output

[alexhambley]

This PR primarily addresses: #37, #34, #33, #28, #17

The snakemake workflow consists of 3 main rules:

• source_ro_crates sources the RO crates and checks their output
• merge_filescreates the merged .ttl file
• create_ro_crate creates the ro-crate-metadata.json file

The example .ttl file consists of all versions of all publicly accessible RO crates on WorkflowHub.