Add horizontal radar appx option

The traditional approach to multi-Doppler synthesis is to ignore
the w component of the Doppler velocity and solve for w via mass
continuity. For the airborne case, there are many steep angle
observations that can add information to the retrieval if the w
component is retained. This is the default mode in SAMURAI.
However, the resulting vertical velocity can be unstable due to
ill-conditioned normal equations just above the horizontal plane.
This option allows users to ignore the w component and perform the
equivalent of a 2-equation solution if desired.
Fix bg interpolation for fine grids

When interpolating from a coarse to fine grid artifacts were
apparent in the gradient fields at the node locations. The maximum
distance to include data was extended to fix this problem, and
Cressman interpolation was added as an option.
Fix qvprime missing data
Update citation for Bell et al. 2012