fix and test transform_vector_cartesian_to_cylinder

[christophlohrmann]

Description of changes:

• Fixes missing sqrt in calculation of vector transformation
• Unit test the function

[jngrad]

Looks like these equations from the failing tests have the wrong unit for the velocity:
https://github.com/espressomd/espresso/blob/python/testsuite/python/observable_cylindricalLB.py#L109-L110
https://github.com/espressomd/espresso/blob/python/testsuite/python/observable_cylindrical.py#L95-L97
Working out v_x and v_y backwards from the expressions in the utils function, I think the correct values should be:

            r = np.sqrt(position[0]**2 + position[1]**2)
            v_y = (r * self.v_phi + position[1] * self.v_r) / r
            v_x = (r * self.v_r - position[1] * v_y) / position[0]

but with these, phi values from the core are different from the reference values by a factor sqrt(2), and have the wrong signs.

[christophlohrmann]

I changed the velocity calculation. Will push in a minute

[jngrad]

Thanks a lot!