(it was alrady the same type, but now the filtering code can be independent on Interval_nt_advanced
(think other interval types, or other types of filters).
* CGAL_error to CGAL_error_msg
* introduced a macro CGAL_error()
* added some words about CGAL_error to the developers manual
* renamed most of assert(x) into CGAL_assertion(x)
* renamed exit(x) with x != 0 , CGAL_assertion(false) and assert(false) into CGAL_error
* CORE left untouched, OpenNL changed