Failure of test_compute_moments on OpenCL backend

[shyams2]

This check tests the output for the moments of a distribution function against its expected values. We assert this error to be lesser than 1e-13. While this test passes when using the CPU backend. It throws the following error on the OpenCL backend:

>       assert(error_rho < 1e-13)
E       assert 4.190951576502503e-08 < 1e-13

We need to check if this is the case on the CUDA backend as well