Altered calculation of connection centres, from midpoint to closest point to each block centre in the connection (potentially two 'centres' for each connection, if the connection face isn't orthogonal to the line joining the block centres)
Bugfix in flux_matrix()- recent changes introduced unnecessary reversal of normal vector
Added connection_nodes() method in mulgrids, with better identifcation of connection nodes- properly preserving node order in the first column of the connection
Altered mulgrid layer_plot() and slice_plot() so that flows are not shown in columns with less than 3 nodes
Bugfix in layer_plot() and slice_plot()- recent changes caused an error if no flows were being plotted
Bugfix in mulgrid optimize(): column areas weren't being updated
Added polygon_centroid() function to geometry module- this is now used to calculate column centres, and should give better results for columns with more than 4 sides.  Also modified polygon_area() function to work similarly
Merge branch 'fluxmatrix' into testing