Add collect function

Fixes #1019.  Thanks Robert Jenssen, who sent such an implementation
years ago.  I added some doctests.  Also fixes upstream issue [1].

[1] https://savannah.gnu.org/bugs/?59014

NEWS

@@ -3,6 +3,7 @@ octsympy 3.1.1+
 
   * New symbolic commands:
 
+          collect
           fplot
 
   * Move repo to https://github.com/gnu-octave/symbolic

inst/@sym/collect.m

@@ -0,0 +1,66 @@
+%% Copyright (C) 2020 Robert Jenssen
+%% Copyright (C) 2023-2024 Colin B. Macdonald
+%%
+%% 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
+%% @deftypemethod  @@sym {@var{e} =} collect (@var{f}, @var{x})
+%% Collect common powers of a term in an expression.
+%%
+%% An example of collecting terms in a polynomial:
+%% @example
+%% @group
+%% syms x y
+%% f = y*x^2 + x*y + x*y^2
+%%   @result{} f = (sym)
+%%        2        2
+%%       x ⋅y + x⋅y  + x⋅y
+%%
+%% collect(f, x)
+%%   @result{} (sym)
+%%        2       ⎛ 2    ⎞
+%%       x ⋅y + x⋅⎝y  + y⎠
+%%
+%% collect(f, y)
+%%   @result{} (sym)
+%%          2     ⎛ 2    ⎞
+%%       x⋅y  + y⋅⎝x  + x⎠
+%% @end group
+%% @end example
+%%
+%% @seealso{@@sym/expand}
+%% @end deftypemethod
+
+
+function e = collect(f, varargin)
+
+  if (nargout > 1)
+    print_usage ();
+  endif
+
+  f = sym(f);
+  for i = 1:length(varargin)
+    varargin{i} = sym(varargin{i});
+  endfor
+
+  e = pycall_sympy__ ('return collect(*_ins)', f, varargin{:});
+
+endfunction
+
+
+%!test syms x y z
+%! f = [x*y + x - 3 + 2*x^2 - z*x^3 + x^3];
+%! assert (logical (collect (f,x) == ((x^3)*(1 - z) + 2*(x^2)  + x*(y + 1) - 3)))