Overhaul contour labeling functions. * scripts/plot/clabel.m: Use 'h', rather than 'retval', to match variables to documentation. Improve performance of input processing by using try/catch block and eliminating for loops. * scripts/plot/private/__clabel__.m: Get X and Y spacing in points from axis rather than assuming 4"x3" plot figure. Fix incorrect determination of axis limits if no contour handle provided. Rename loop vars i1, j1 to i,j. Performance improvement by using bsxfun over repmat. Use find to replace while loop (slow). Keep label rotation in the range [-90, 90] for readability.