Linear solver wrappers

[brbass]

Summary

• Adds a general-purpose linear solver interface with Hypre (distributed, iterative) and Eigen (local, direct) backends.
• Renames the SPHERAL_INCLUDE_SUNDIALS CMake option to SPHERAL_INCLUDE_SOLVERS.
• Adds tests for solvers.

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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.