Add semimajor axis to MPCORB table

[sgreenstreet]

I see it listed as an issue here already, but since I've heard requests from several people about this, I would like to second the request to have the semimajor axis added to the MPCORB table in the data schema. I've also heard that this is the plan, but I wanted to mention it again to make sure it's noted that there are multiple requests for this addition.

[hhsieh00]

Thirded

[mschwamb]

It's been flagged in other ways, but it's good to flag this explicitly here too. Thanks for posting.

Note a in in the DPDD - it's q that's not actually in there. It's a flagged issue for cometary and hyberbolic orbits that q is better so q needs to be added with a being clarified that it will be NULL if not defined due to the orbit.