[doc] updated Reference guide : added section of 3D computation on a regular grid
[test] updated test_100U_1N trace due to recent changes in 1D section of distribution
[test] updated test_4_3. The second variable should be a mixture and not a random mixture
[test] minor changes with test_2U_1T
[test] minor changes in test_4_3
[src] In computePDFOnGrids, replaced the gaussian approx by the call oh internal method
[test] minor changes in some tests
[test] added test for evaluations of pdf on a regular grid with analytical results
[test] added test for evaluations of pdf on a regular grid with analytical results in 3D (test_4_3)
[val] added 'big data' validation data for pdf comparisons