gas-model: add R compressibility script

It annoyed me that we hand-waved a bit about how the virial factors were
actually computed in the gas-model.c file, so here's an actual R script
that computes them and plots the results.

You can run it with (for example):

    R --vanilla < compressibility.r

and it will generate a Rplots.pdf of the plots, and the coefficients
will be shown on stdout.

The result actually differs in insignificant ways from the values that
Lubomir computed, which is likely just due to tools.  I used R, Lubomir
seems to have used

    http://polynomialregression.drque.net/online.php

but the actual curve is pretty much the same.

NOTE! R is not entirely happy about the non-linear fit of the Helium
curve: the fit is *so* precise that it failes the R relative-offset
convergence criterion.  That is apparently generally a sign of
artificial data.

That is probably because Lubomir generated them from the linear mix of
two polynomial fits, rather than a linear mix of the original data.  But
maybe the original data was artificial?

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

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