Save dt to Sidre and restore on restart

[white238]

Edit: This does actually bring up a good point in your thorough comment. We are setting time and cycle but not dt (via MFEMSidreDataCollection::SetTimeStep). @jamiebramwell, does the dt every change from the input file value?

Yes - dt can change from the input deck value.

Originally posted by @jamiebramwell in https://github.com/LLNL/serac/issues/717#issuecomment-1154125795