Fix Wilkinson formula term sorting to match MATLAB left-to-right hierarchy

[Pasta-coder]

Fixes #395

When i was integrating parseWilkinsonFormula into the ClassificationGAM class to replace its hardcoded formula parsing, a unit test revealed a discrepancy in how formula terms are sorted compared to MATLAB.

When expanding the crossing operator (*), Octave currently sorts the binary schema matrix in ascending order. Because 0 is less than 1, sortrows places variables with later indices before earlier ones.

for the following MATLAB script

% MATLAB script to verify formula expansion and sorting
x1 = rand(10,1); x2 = rand(10,1); x3 = rand(10,1); Y = rand(10,1);
T = table(x1, x2, x3, Y);

mdl = fitlm(T, 'Y ~ x1 * x2 * x3');

expected_terms = [
0, 0, 0, 0; % Intercept
1, 0, 0, 0; % x1
0, 1, 0, 0; % x2
0, 0, 1, 0; % x3
1, 1, 0, 0; % x1:x2
1, 0, 1, 0; % x1:x3
0, 1, 1, 0; % x2:x3
1, 1, 1, 0 % x1:x2:x3
];

assert(isequal(mdl.Formula.Terms, expected_terms), 'Matrix sorting does not match expected output!');
disp('SUCCESS: The matrix is correctly sorted left-to-right!');

MATLAB output

>> untitled4
SUCCESS: The matrix is correctly sorted left-to-right!

Octave before FIX

octave:43> schema = parseWilkinsonFormula('Y ~ x1 * x2 * x3', 'matrix');
expected_terms = [
0, 0, 0, 0; % Intercept
0, 1, 0, 0; % x1
0, 0, 1, 0; % x2
0, 0, 0, 1; % x3
0, 1, 1, 0; % x1:x2
0, 1, 0, 1; % x1:x3
0, 0, 1, 1; % x2:x3
0, 1, 1, 1 % x1:x2:x3
];
assert (schema.Terms, expected_terms);
disp('SUCCESS: The matrix is correctly sorted left-to-right!');
error: ASSERT errors for: assert (schema.Terms,expected_terms)

Location | Observed | Expected | Reason
(2,2) 0 1 Abs err 1 exceeds tol 0 by 1
(4,2) 1 0 Abs err 1 exceeds tol 0 by 1
(5,2) 0 1 Abs err 1 exceeds tol 0 by 1
(7,2) 1 0 Abs err 1 exceeds tol 0 by 1
(2,4) 1 0 Abs err 1 exceeds tol 0 by 1
(4,4) 0 1 Abs err 1 exceeds tol 0 by 1
(5,4) 1 0 Abs err 1 exceeds tol 0 by 1
(7,4) 0 1 Abs err 1 exceeds tol 0 by 1

after FIX

octave:47> clear classes; rehash;
octave:48> schema = parseWilkinsonFormula('Y ~ x1 * x2 * x3', 'matrix');
expected_terms = [
  0, 0, 0, 0;  % Intercept
  0, 1, 0, 0;  % x1
  0, 0, 1, 0;  % x2
  0, 0, 0, 1;  % x3
  0, 1, 1, 0;  % x1:x2
  0, 1, 0, 1;  % x1:x3
  0, 0, 1, 1;  % x2:x3
  0, 1, 1, 1   % x1:x2:x3
];
assert (schema.Terms, expected_terms);
disp('SUCCESS: The matrix is correctly sorted left-to-right!');
SUCCESS: The matrix is correctly sorted left-to-right!
octave:52> clear classes; rehash;
octave:53> test parseWilkinsonFormula
PASSES 75 out of 75 tests
octave:54>