SCR compression

Nobody has claimed this yet.

We should consider adding compression in SCR. We mention wanting to do it in https://computing.llnl.gov/projects/scalable-checkpoint-restart-for-mpi and in src/scr_io.c:

```C
/* TODO: could perhaps use O_DIRECT here as an optimization */
/* TODO: could apply compression/decompression here */
/* copy src_file (full path) to dest_path and return new full path in dest_file */
int scr_file_copy(
const char* src_file,
const char* dst_file,
unsigned long buf_size,
uLong* crc)
{
...
```
I can see cases where it would be beneficial, and cases where it wouldn't. If we did it, I'd recommend we use zstandard (https://github.com/facebook/zstd) which is currently the best compressor/decompressor out there.

Start by reading src/scr_io.c, especially scr_file_copy, and the linked SCR project description to understand where checkpoint data is copied. Define when compression should apply and how zstandard would fit before implementing it; done requires an agreed design and validation of both beneficial and non-beneficial cases.

Written by the indexing model from the issue text.