Not enough options or simplicity to control diag/restart dump times

[igirault]

Today we always need to define a timestamps array, that most of the time is based on a call to numpy's linspace or arrange, so with a constant time period. Why just not specify this time period directly instead of an array of timestamps ?

With the adaptive time-step coming, you sometimes also want to dump every N iterations instead of a regular time period.

This calls for two new options in the diag/restart params, time_period and step_period.

[nicolasaunai]

I I've converted this to a discussion since it's not exactly ready for an issue.
I'm not so sure I understand the reason for wanting cadenced diagnostics while having variable time step. That will produce irregular time sampling only because the CFL changes the time step. I'd think that if the user wants irregularly space diagnostics in time it's for physical reasons, like refining time resolution over some intervals for averaging stuff or just having the opportunity to see high frequency stuff around a specific time.

Open do to dscussion on that matter