Frontal volume detection: Mountain mask and external smooth field

This merge entails only changes to the frontal volume detections in detect.py Both the large-scale and the small-scale frontal volume detection can now take (optional kwarg) an external boolean field where True corresponds to points that are masked out Additionally, the small-scale can take an external smooth tfps field that is then used in the second step of the detection. For <> this falls back to the 2D smoothing of dynfor.utils.smooth_xy