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

removed non-functional dead code: preCompileSelector and benchmark.cpp; fixed clang-tidy -checks=clang-analyzer-*

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

Added further usefull warnings and changed xerus to comply. In particular in accordance with the C++CoreGuidelines a warning is issued for C-Style casts.

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

Fixed factor handling in entrywise product, fixed soft treshold function, added test case (though incomplete) for largest entry determination

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