Add preconditioner interface

[KakeruUeda]

Description

This adds a preconditioner base class and refactors SystemSolver to use it, removing the current coupling to LinSolverDirect and enabling custom preconditioner implementations.

Closes #114

cc @shakedregev

Proposed changes

Added a Preconditioner base class with the following methods:

• setup()
• apply() (calls the preconditioner)
• reset() (for updated matrix values with the same sparsity pattern)

Checklist

• All tests pass (make test and make test_install per testing instructions). Code tested on
  • CPU backend
  • CUDA backend
  • HIP backend
• I have manually run the non-experimental examples and verified that residuals are close to machine precision. (In your build directory run:
  ./examples/<your_example>.exe -h to get instructions how to run examples). Code tested on:
  • CPU backend
  • CUDA backend
  • HIP backend
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows Re::Solve style guidelines.
• [N/A] There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.

Further comments

[shakedregev]

@KakeruUeda - it seems some tests are not passing. I will wait before I review this.

[pelesh]

Clean implementation consistent with the rest of the code. All tests pass.