ugly scans and compares URLs became singularized
fix SNP pattern not to allow wrong symbols on far sides of flanks (left flank could contain any symbols farther than +minimal_flank_size+; right flank couldn't but is marked with \b too not to forget during future improvements where SNP sequence may be not the last part of string); also made +valid_SNP_line?+ a class method mostly to made explicit which method is called during validation (because it isn't obvious in which context validation is called)