move actionAngleIsochroneApprox to top level, and necessary changes
start on new documentation of potentials
densities of potentials in galpy
add reference labels
add new documentation figures
add comment about dblexp denity computed with the Poisson equation vs. analytical
add docs on props of close-to-circular orbits
finish new documentation of potential functions (fixes #106) and add documentation on adding potentials to the galpy framework (fixes #22)
edit dev README with updated function names