add option to provide dxdv integration input in rectangular coordinates (working toward #174)
add option to provide dxdv integration input in rectangular coordinates (working toward #174)

jobovy/galpy,followthesheep/galpy,followthesheep/galpy,jobovy/galpy,jobovy/galpy,jobovy/galpy,followthesheep/galpy,followthesheep/galpy