Add method to convert a sym to a pyobject

This is for use with the Pytave project.

inst/@sym/pyobject.m

@@ -0,0 +1,54 @@
+%% Copyright (C) 2016 Colin B. Macdonald
+%%
+%% This file is part of OctSymPy.
+%%
+%% OctSymPy is free software; you can redistribute it and/or modify
+%% it under the terms of the GNU General Public License as published
+%% by the Free Software Foundation; either version 3 of the License,
+%% or (at your option) any later version.
+%%
+%% This software is distributed in the hope that it will be useful,
+%% but WITHOUT ANY WARRANTY; without even the implied warranty
+%% of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See
+%% the GNU General Public License for more details.
+%%
+%% You should have received a copy of the GNU General Public
+%% License along with this software; see the file COPYING.
+%% If not, see <http://www.gnu.org/licenses/>.
+
+%% -*- texinfo -*-
+%% @documentencoding UTF-8
+%% @defmethod @@sym pyobject (@var{x})
+%% Convert/cast a sym object to a pyobject.
+%%
+%% Example:
+%% @example
+%% @group
+%% syms x
+%% f = 2*sin(x/2);
+%% pyobject(f)
+%%   @result{} ans = [pyobject ...]
+%%
+%%       2*sin(x/2)
+%%
+%% @end group
+%% @end example
+%% @seealso{@@sym/char, py, @@pyobject/pyobject}
+%% @end defmethod
+
+
+function y = pyobject (x)
+
+  y = py.sympy.S (char (x));
+
+  % Above is nice because it needs no imports.  But downside is some
+  % things like NonElementaryIntegral and MatrixElement will fail.  See
+  % e.g., commit 51a80384d5caea0db211e452d39f0b4f6b3778cc
+
+  % Alternatively, with many imports (see python_header.py), this works:
+  %y = pyeval (char (x));
+
+end
+
+
+%!assert (isa (pyobject (sym ('x')), 'pyobject'))