The acceptance criteria ask for numbers rather than claims, and every row but
the three still-unbuilt ones has been measured. Splits validation into the
Orekit comparison and the analytic invariants, and gives the performance table
step counts alongside times so a ratio of times can be read as per-step cost.
The three invariant tests and the MSIS drag comparison now print what they
achieved, so the readme's figures can be reproduced by running them.
Also corrects the ordinary-diffeq version in the foundations table, and the
claim that the fixture generator produces observations, which it does not yet.
connor