Optionally only estimate depth of features

[stephanietsuei]

The current representation of features in the state is xc = [X/Z, Y/Z, log(Z)]; this estimates each feature's depth and direction and uses 3 states per feature. The original version of Corvis only estimated log(Z) and fixed the directions at the feature's first detection, using only 1 state per feature. Extend XIVO so that users can choose between these.