timing_model: call setup() after reading parfile.
astrometry: use astropy Angles and Units internally.
Comment edits.
timing_model: exception class for missing required params.
spindown: add some parameter-checking
Add vim .swp files to gitignore.
spindown: use pepoch
timing_model: add source name
timing_model: add param help functions.
Minor variable renaming, comment edits, remove some test statments.
astrometry: add solar system geometric delay calc (from Jing's note)