- add CombinatorialSpecSet in spack.util.spec_set module.
- class is iterable and encaspulated YAML parsing and validation.
- Adjust YAML format to be more generic
- YAML spec-set format now has a `matrix` section, which can contain
multiple lists of specs, generated different ways. Including:
- specs: a raw list of specs.
- packages: a list of package names and versions
- compilers: a list of compiler names and versions
- All of the elements of `matrix` are dimensions for the build matrix;
we take the cartesian product of these lists of specs to generate a
build matrix. This means we can add things like [^mpich, ^openmpi]
to get builds with different MPI versions. It also means we can
multiply the build matrix out with lots of different parameters.
- Add a schema format for spec-sets
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