miss_feature--PDI--pack_and_unpack_data

SIONlib does not have a concept of datatypes; its read and write functions have signatures similar to fread() and fwrite() in the C standard library. For the SIONlib plugin, I would go with a strategy where sparse data is packed into a buffer before it is written to disk. Similarly, upon reading, the data would be moved out of the file, into a dense buffer and then unpacked into the sparse buffer in the application. I noticed that there is already a "pack" function (sparse -> dense) called tcopy(). However, it is not exported to the plugins as far as I can tell and there also is no "unpack" function (dense -> sparse). I think it would make sense for the core library to provide these functions to the plugins.