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

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

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

Tensor now decides based on sparsity estimates whether the result of contraction is sparse. Also clean up of header includes