Maths: added function for computing the Determinant of a 3x3

Signed-off-by: Nicola Vigano <nicola.vigano@esrf.fr>

zUtil_Maths/gtMathsDet3x3.m

+function d = gtMathsDet3x3(m)
+
+    d = m(1, 1, :) .* (m(2, 2, :) .* m(3, 3, :) - m(2, 3, :) .* m(3, 2, :)) ...
+        - m(1, 2, :) .* (m(2, 1, :) .* m(3, 3, :) - m(2, 3, :) .* m(3, 1, :)) ...
+        + m(1, 3, :) .* (m(2, 1, :) .* m(3, 2, :) - m(2, 2, :) .* m(3, 1, :));
+end
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