add snapshot and interpsnapshot potentials to the docs (#212)
rename SnapshotPotential in authors and readme (#212)
edit documentation of SnapshotRZ and InterpSnapshotRZPotential (#212) [ci skip]
test of epifreq (#216)
tweaks to epifreq interpolation; fix sign of second derivatives
test of verticalfreq (#216)
tests of epifreq and verticalfreq; fix normalization of InterpSnapshotRZPotential
tests of snapshotrz and interpsnapshotrz with no pynbody loaded