add timestep drop detector

[BenWibking]

Describe the proposal
The code should detect (and halt) if the timestep drops by several orders of magnitude, similar to the dt_cutoff parameter in Castro (https://amrex-astro.github.io/Castro/docs/timestepping.html#additional-controls):

"The dt_cutoff parameter will force the code to abort if the timestep ever drops below 1e-12 of the current time. This is a safety feature—if the code hits such a small value, then something likely went wrong in the simulation"

Additional context
This is probably due to a lack of strict entropy stability. This is still observed in some simulations with NSCBC boundary conditions.