- added registry for print_context classes (use print_context_class_info::dump_hierar...

- added registry for print_context classes (use print_context_class_info::dump_hierarchy()
  to show the class hierarchy tree)
- the default output format is now handled by the print_dflt type, but
  print_context remains the base class for the print context hierarchy
- you can override the default output format for functions on a per-function
  basis by specifying one or multiple print_func<C>() (C is a print_context
  type) function options (see inifcns.cpp/abs() for an example)
- better LaTeX and C source output for abs()