* Add package for canu
There's a top-level perl script, which is touched up to use a
spack-built perl instead of `#!/usr/bin/env perl`. It's
self-contained, it uses `FindBin` to locate it's libaries.
* Add dependency on gnuplot
I noticed a hidden mention of the use of gnuplot in the docs.
This adds the dependency.
* Tighten dependencies, flake8 cleanup
gnuplot is a run dependency.
jdk is a run dependency.
perl is a run dependency.
Make flake8 happy.
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