Add hypre and eigen sparse solvers

[brbass]

Summary

• This PR is a (refactoring, bugfix, feature, something else)
• It does the following:
  • Modifies / refactors (class or method) (how?)
  • Fixes (issue number(s))
  • Adds (specific feature) at the request of (project or person)

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ToDo :

• Annotate RELEASE_NOTES.md with notable changes.
• Create LLNLSpheral PR pointing at this branch. (PR#)
• LLNLSpheral PR has passed all tests.