Fix dynamic core CoreData initialization

[Francyrad]

This is a small follow-up patch for geodynamics#6924.

It avoids copying CoreData from the CoreStatistics postprocessor on fresh runs, where the postprocessor data has not been initialized yet. In that path, core_data.H is initialized to 0.. On restart, the restored postprocessor CoreData is still used, and the current dt, H, and derived core quantities are recomputed before continuing.

@gassmoeller, does this match the initialization behavior you had in mind, and do you think it fully resolves the CoreData issue you pointed out?

Checks run locally:

• Debug build: passed
• No-unity debug build: passed
• Dynamic core restart smoke test: passed
• dynamic_core: passed
• dynamic_core_fully_solid: passed
• dynamic_core_fully_molten: only a local last-digit statistics diff, no SIGFPE