Dynare++ tensor library: modernize the Symmetry class

We now use a initializer list constructor for creating symmetries of the form
$y^n$, $y^n u^m$, $y^nu^m\sigma^k$.

The constructor taking a single integer is used to initialize a symmetry of a
given length.

Similar changes are made to IntSequence.

This behavior is similar to std::vector.
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