I want to install a single package, how do I do this?

This is easy! Suppose you wish to install the package foo, all you need to do is

./dorsal.sh install-package foo

Dorsal will then look for foo.package in the 'packages' folder and follow its instructions.

The only caveat when attempting to doing this is that Dorsal is not aware about the dependencies for a given package. This means that while doing this for a package lower down the dependency chain (e.g. petsc) works well, doing this for a package higher up the dependency chain (e.g. DOLFIN) will fail unless all its prerequisites have been previously installed correctly.