#2785 dynamics package: plotting fails on evolution() and other functions

[Yasuaki Honda]

In dynamics package, functions evolution(), evolution2d(), orbits() and staircase() failes to plot when [plot_format, gnuplot] is used.

Root cause: these four functions generates xlabel and/or ylabel from the specified expression. The types of values for xlabel / ylabel is atom, not string. Then some strange value is generated in maxout.gnuplot. Gnuplot fails to treat such a file.

Fix: in generating xlabel / ylabel, sconcat should be used instead of concat. For some places, it is necessary to use string() to convert the value to string type.

See attached patch in detail.

[Yasuaki Honda]

patch applied as commit 5dfc13.