set MG_tolerance_abs to numeric_limits min

[AlexanderSinn]

If MG_tolerance_abs would go lower than numeric_limits::min(), the MG solver would fail to converge due to loss in precision. Such a scenario could occur if the source term is zero but the initial guess is larger than zero.

Note in my testing this did not stop it from crashing in single precision.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
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• Constified (All that can be const is const)
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• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable

[SeverinDiederichs]

Thank you for this safety guard.