more frequency interpolation modes for beams

[aewallwi]

uvbeam, as implemented, uses interp1d with default args. There are a number of instances where more customized frequency interpolation may be desirable. Two primary modes that may be desirable include:

1. Nearest neighbor inerpolation -- allows for monochromatic beams.
2. Fourier interpolation -- interpolate using fourier modes specified by the DFT of the beam along each LoS (avoids introducing frequency-interpolation artifacts into the delay-kernel).

[bhazelton]

Nearest neighbor and nonlinear interpolation (any accepted by interp1d) were added in #547.