Try to plot Lambda and Lambda'

This commit adds a _lot_ of data so that a new TODO entry to tidy that
bloody mess up was finally due.

Basically, there is a data file *.xy{z,}, a gnuplot script make_*_plot and
the resulting PNG - and yes, I chose to commit all those files.

A new program, lambda-prime-seq, had to be added to generate the
lambda_prime.xyz data file. It prints out all Lambda' number for a given T
from 1 <= n <= #T instead of the #sqrt(T). This makes the graph of pi(n)
show up in the x-y plane of lambda_prime_plot.png and justifies that I gave
it the name T*n triangle in my paper.