Fixed dt value for some tests
updated documentation examples: fixed dt=0 in head of documents, dt=1e-3 in examples to run fast
changed test-all : comment test_check in order to not run PANDA tests with R CMD check; the tests are checked by travis every push
in tests, removed test of mu_parameter because of too small values
added an example with non null extinction in fit_bd
updated plots: x axis is negative