Mint DOIs for pyspedas (all versions) and each individual release

[jameswilburlewis]

TBD: Find some guidance on best practices for minting DOIs for software. Also, how would we include the DOI for a specific release inside that release itself (e.g. in an "about" or "version" routine)? Since I think you don't get the DOI after uploading the files, and it's probably not good practice to modify the release after the DOI is minted....