Consider adding HDF5 compression to EmpCylSL cache files

[The9Cat]

The gain may be small, but worth checking.

[The9Cat]

I tried this: got a compression factor of a bit more than 1.2. Trivial code change but maybe not worth it?

[The9Cat]

This is useful tidbit of information...

I took a disk cache file with 256x128 grids and used h5repack to compress it to the max. I used the command:

h5repack -v -f GZIP=9  --layout CHUNK=256x128 eof.uncompressed eof.compressed

That achieved a compression factor of 1.42, so not bad. Changing the compression parameter to 5 does not change the compression. No additional code needed, so I will bake these parameters into PR #183

[michael-petersen]

This is great, good note as well. Closing as we have merged this in to main.