turn travis back on
mass for axisymmetric potentials and test
use different non-axi potential in mass test; properly put isNonAxi for MovingObjectPotential
rename test
explicitly implement mass for KeplerPotential
few tests for spherically symmetric masses
some tests for TwoPowerSphericalPotentials, fix KeplerPotential mass
test for NFW mass, #128
test that explicitly implemented masses agree with the integral of the density
test higher-order derivatives
rm phi keyword in mass
consistent _mass interface