Draft: Adding a unit test for the chebyshev filtering.
Description
Adding a unit test for the chebyshev filtering, testing some properties of the Chebyshev polynomials. This is used for checking that T_{2n}(x) = 2T_n(x)-1.
News snippet
Adding a unit test for the chebyshev filtering
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.