* Add the rockstar package
* Add rockstar to yt.
* Correct signature of prep_yt.
* Correct rockstar package installation process
Needed to change the way the necessary prefixes are found as well as
how the completed package is copied.
There should probably be a way to do the copying in a more 'spack' way.
* Add license
* Add description.
* Set no hdf5 variant as default.
* Use copy_tree for copying files
* Use MakefilePackage
* Address concerns from @adamjstewart
* Fix flake8 errors.
