get_distances_of_nodes_to_point suppress warnings

get_distances_of_nodes_to_point in base.py used to be unable to handle
cases where the point was on a node without warnings. It now can. The
aspect of the central node is now returned as 0.
Added imshow_core_node_grid

Added imshow_core_node_grid() to plot.imshow.
ID conversion functions in sgrid

Added functions to return the “real” ID from position in an array of
all interior nodes, and vice versa, to structured grid.
facet_finder

This util searches for planar facets adjacent to fault traces on DEMs.
It is for data analysis; you’d be crazy to run this on a looped model.