rename data to io...still need to do some reorganization
reorganize io subpackage
sketch for new RV to UVW and UVW to RV functions
reorder imports
smarter table reading
remove old combine catalogs test
added _x attribute to particle collecton for fast access to all 6 phase-space coordinates
fix test and _r accessing
added .to() method
test .to() method on particle collection
added function to convert a table to a PC
updated orphan particles
implemented base class for ParticleCollection and OrbitCollection
subclass from dynamicalbase
orbitcollection now a dynamicalbase subclass
use nparticles attribute
added test for to() in orbitcollection
add getitem to get a particle collection from orbits
fix slicing, add test
fix table to particles function