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^2(x)-1

.

News snippet

Adding a unit test for the chebyshev filtering

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.