SPICE - rename spin and repoint subsec fields to ms

[subagonsouth]

          `subsec` has a lot of meanings and is often not ms. Could you use something like `repoint_end_ms` instead?

Originally posted by @maxinelasp in #1500 (comment)

Review this once we get example spin and repoint files from the MOC. The implementation is somewhat dependent on if the CSV includes the column names on the first line or not.