dynamically using LDL, LU or Cholesky solver in dense matrix solve to speed up calculations (relates #194)

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