Unit tests failing on changing arrayfire backend

[amanabt]

Commit: 90e4f16

Issue: The functions test_wave_equation_2d.py::test_A_matrix and test_waveEqn.py::test_volume_integral_flux are failing when the backend is set to opencl. They pass when the backend is cpu or cuda.

[amanabt]

@Balavarun5
Please resolve the issue with test_waveEqn.py::test_volume_integral_flux.

[amanabt]

The reference values for the A matrix were being calculated using LGL points with accuracy only till 4th place of decimal. Correcting it results in the error to reduce to machine error.