Added mulgrid methods for subdividing columns with more than 4 sides; in geometry polygon_centroid(), added check for columns with <3 sides
In mulgrid columns_to_tri_quad(), added columns parameter, to allow application to only part of the grid
In mulgrid fit_surface(), allowed fitting to columns with more than four sides
Renamed mulgrid columns_to_tri_quad() to decompose_columns(), and altered it to modify the geometry in-place instead of returning a new geometry- to make it more consistent with other methods, e.g. refine(), fit_surface() etc.
Added chars parameter to mulgrid decompose_columns() method
Added documentation for mulgrid decompose_columns() method