- rule `doc` in ./Makefile depended on `doc/parseDoxytags` which had no rule
  in ./Makefile
- add missing `-std=c++11` flag to `$(CXX)`-calls in ./doc/Makefile
- replaced .FORCE rules by .PHONY declarations

The declaration of those variables as static const class members caused
problems in codes, that did not define exactly those variables in their
code. This behaviour was highly unexpected.

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.

Closes #142 Tensor modify_diag_elements now support arbitary degree, SFINAE member function check now compatible with C++14, C++1y now used for compilation, therby dropped support for gcc < 4.8