add an init_shrink_timestep parameter

[BenWibking]

We should add a runtime parameter than shrinks the initial timestep by a given factor in order to avoid start-up error/stability issues.

In Castro, this is done with this parameter: https://amrex-astro.github.io/Castro/docs/timestepping.html#additional-controls

cc @psharda

[psharda]

Is it not better to directly specify the initial timestep, instead of defining a scaling factor? In FLASH, we use a runtime parameter called dt_init (similar to what you have in

quokka/src/HydroLeblanc/test_hydro_leblanc.cpp

Line 361 in d17b5e4

sim.initDt_ = initial_dt;

)

[BenWibking]

The initial timestep is problem dependent, whereas the scaling factor of 0.1 or 0.01 might work for a large range of problems.

[BenWibking]

Implemented in #1559.