using xerus::misc::randomEngine per default, better randomness in unittests, fixed a faulty size_t cast in cholmod wrapper (still more bugs found by better randomness...)

less namespace pollution (relates #182): all exported and used macros now use the XERUS_ prefix. LogFlag now in xerus::misc::internal

Renamed SelectedFunction to basicArray support and dropped the CBlas dependency, using only standard c++ operation, resulting in no measurable time increase

Closes #98 approx_equal no a tensor function and using the relative difference in the frobenious norm, some test cases adjusted