Filled contours now use more of the available colormap range than
previously.

This change should have no effect on any plot, but it avoids
warnings that can arise when an inf occurs under a mask.

Previously, although vmin, vmax could be specified manually
in a norm used to initialize a ContourSet, the values would
be used only if both were specified.  Now one can set either
or both of vmin, vmax in the norm, and they will take precedence
over the default.

This changeset also simplifies and clarifies the vmin, vmax
explanation and handling for pcolor and pcolormesh.

…ntour

Formerly, colorbar would not place ticks at contour levels outside the
vmin-vmax range, and would not add line contour marks at such levels.