Load & Save Custom for large files

Alternative methods to save and load large variables still using -v7 mode

Matlab's 'save' command works with large variables (> 2^31) with option -v7.3, but it seems compression with -v7.3 is less efficient than -v7. So this script splits the large variables in multiple files still using -v7 compression. Multiple file saved with this script should be then loaded with load_custom. The syntax is identical to that of the Matlab save command.