Implemented the function append_cxi() to image_io.c, which has been added to sp_image_write() for the CXIDB file format. The implementation contains functionality to create new file, append entry to existing file, append image to existing entry, and append data to existing image (see implementation notes). The function will write axes attributes to the stacks and iteration number for each image folder. Thus, the flag argument used by sp_image_write() was changed to 64-bit integer (long long).
Merge branch 'master' of https://github.com/FilipeMaia/libspimage