doc pages for new Python function wrappers

examples/python/in.python.wrap

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+# 3d Lennard-Jones melt with equal- and atom-style variables which
+#   use a Python function wrapper in their formulas
+
+variable        x index 5
+variable        y index 5
+variable        z index 5
+
+units           lj
+atom_style      atomic
+
+lattice         fcc 0.8442
+region          box block 0 $x 0 $y 0 $z
+create_box      1 box
+create_atoms    1 box
+mass            1 1.0
+
+velocity        all create 1.44 87287 loop geom
+
+pair_style      lj/cut 2.5
+pair_coeff      1 1 1.0 1.0 2.5
+
+neighbor        0.3 bin
+neigh_modify    delay 0 every 20 check no
+
+fix             1 all nve
+
+variable        foo python truncate
+python          truncate return v_foo input 1 v_pyarg1 format fi here """
+def truncate(x):
+  return int(x)
+"""
+variable        pyarg1 internal 0.0
+
+variable        scalar equal py_foo(4.5)
+print           "TRUNCATE ${scalar}"
+
+variable        xtrunc atom py_foo(x)
+variable        ytrunc atom py_foo(y)
+variable        ztrunc atom py_foo(z)
+
+# examine dump file to see truncated xyz coords of each atom
+
+#dump            1 all custom 100 tmp.dump id x y z
+dump            1 all custom 100 tmp.dump id x y z v_xtrunc v_ytrunc v_ztrunc
+
+run             100