[examples] made plotting optional
[scripts/tidyup.sh] generalized deletion of *.so files
[tools/plot_test] covariances changed to test special and general case
[pmc_test] added "test_invalid_cov"
[density/mixture] removed support for iteration and item access
[tools/indicator] cover full code with test cases
[scripts] added quick_test.sh (run all tests that are not marked slow)