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

REQUIRE now throws error instead of fatal, introduced INTERNAL_CHECK and changed some macro names to have a XERUS_ prefix

Added implementation for perform_trace() for sparse Tensors, added a test case for it and fixed a bug in fixed index handling in TN

fixed compilation with clang and always creating the log buffers for applications that use our library with different flags