not tested yet.

code runs without segfaults. I still need to test the correctness.

very simple functionality, but I gain two lines per value in the useful
file, which I consider a big deal.

This reverts commit b4bbc44b.

This reverts commit 6c803fa1.

This reverts commit f843c813.

the "simname_particles.h5" file is now only opened once, and then
samples are written the same as before.
some sanity checks of the velocity and acceleration data should still be
performed.

for linking against some particular libraries, it's necessary to add
"extra_libraries" such as '-Wl,--start-group'.
Therefore when the compile command is now constructed, "extra_libraries"
are checked to see whether they start with '-' or '/' (for static
libraries given as full file names).