Converted order-dep from Gnuplot to Asymptote.

Converted order-dep from Gnuplot to Asymptote.

This lead to some changes in the Asymptote build system:
  * New pyfit approach: site_init.link_pyfit() over old Alias('pyfit').
    The Alias approach made path extraction ugly and brittle.
    The new approach mirrors link_wtk_graph().
  * Adjust graphFile() and pyfit to allow single column (i.e. implicit
    x, explicit y) datafiles.
  * Adjust pyfit to handle single-parameter fitting.

I also increased the horizontal spacing between the v-dep-sd histogram
insets.