revert to first-order space+time flux when RK2 flux fails (#221)

* use Roe-average wavespeeds

* fix bug in roe-averaged sound speed

* use LLF solver with first-order flux

* revert to first-order space+time flux

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* cherry-pick fix for SphericalCollapse

* remove Riemann solver template parameter

* fix typo

* remove LLF solver (unused)

* Update src/RadhydroSimulation.hpp

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* Update src/RadhydroSimulation.hpp

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* update HydroHighMach tolerance

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Mark Krumholz <[email protected]>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>