Large error in calculation of double integral.

[amanabt]

Commit: 3b42c3f

Issue:
The error in the calculation of double integral
is of the order of in the test_wave_equation_2d.py::test_A_matrix unit test.

As of now, the cause of the issue is not known.

[amanabt]

The reference values were calculated using the LGL points with an accuracy of 1e-4,
whereas, the unit test was using LGL points with an accuracy of machine error. The
difference in accuracy of the LGL points lead to large errors b/w reference and test
A matrix.

Recalculating the reference values using LGL points with higher accuracy reduced the
error b/w reference and test A matrix to machine error.

Issue fixed in PR #44