Trimming down ephemeris file

[hannorein]

I wrote a small program to generate smaller planet ephemeris files, e.g. linux_p2023p2031.440. This way I don't need to download/copy the full 100MB linux_p1550p2650.440 file if I only need a small baseline. For reference, one year of ephemeris data takes up only about 100KB. This could be useful if someone would want to include a datafile in a package or container.

Would this be of interest to include in ASSIST? (if so, don't merge it yet, I'll clean up the code beforehand)