- added stream manipulators index_dimensions and no_index_dimensions

- added stream manipulators index_dimensions and no_index_dimensions
- using print() and print_context (and derived classes) in user code is now
  deprecated in favor of the manipulators
- print_options moved from flags.h to print.h
- a symbol that appears multiple times in archives but is not listed in the
  sym_lst passed to unarchive_ex() now receives the same serial number in
  all occurrences, so GiNaC will treat it as one and the same symbol; this
  made it necessary to remove the "const" from the second argument of
  basic::unarchive() and the unarchiving constructor