fix indexing of points outside of the grid
tests of interpRZPotential's potential and forces
some tests with interpolated potentials, but always within the grid
tests for other cases of 2D, non-axisymmetric potentials
tests for other cases of 2D, non-axisymmetric potentials
test Rphi deriv, implement this as an instance function of planarPotential
clean up DoubleExponentialDiskPotential