quad: examples: fix quadrature namespace in remaining examples

TODO: convergence test for serial MLSDC with advection-diffusion example
      does not work yet:

    PFASST/tests/examples/advection_diffusion/test_advection_diffusion.cpp:72: Failure
    Value of: err
    Expected: contains 4 values, where each value and its corresponding value in { 8e-11, 8e-11, 8e-11, 8e-11 } double less
      Actual: { 7.56664e-11, 1.46309e-10, 2.12063e-10, 2.73565e-10 }, where the value pair (1.46309e-10, 8e-11) at index #1 don't match

[skip ci]