Large .geogram files

[BrunoLevy]

• FIXED cannot write a .geogram file if one of the arrays is larger than 4 Gb
  • Linux
  • Mac
  • Windows
• Write a small test program and test it on the three platforms
• distinguish and handle properly .geogram files written in GARGANTUA and in standard mode
• optimize (for now, converts each read index_t one by one ...), and aways store arrays in 32 bits if they fit.
• we may have a problem with NO_INDEX that has a different representation in GARGANTUA and in standard mode

Changes summary:

In geogram/third_party/zlib/zconf.h

#if defined(__APPLE__) || defined(_WIN32) || defined(_WIN64)
#define _LARGEFILE64_SOURCE 1
#define _LFS64_LARGEFILE 1
#define _FILE_OFFSET_BITS 64
#endif

• On MacOS, got z_off64_t (and off64_t) undefined type name -> under MacOS, file IO is always 64 bits, #define off64_t off_t and #define lseek64 lseek.
• On Windows, in geofile.cpp, missing type specifier (does not know ssize_t it seems) -> seems to be OK when I defined it.