Update dynamic_core.cc

[Francyrad]

The previous code would halt if no solution was found for the inner core, preventing the simulation from proceeding. This limitation made it impossible to observe the Earth's mantle evolution during cooling when the inner core had not yet formed.

The updated code addresses this issue. Now, if the inner core has not formed, the simulation continues, allowing the core-mantle boundary (CMB) to cool. If the inner core eventually forms during the simulation, its radius will be recorded in the statistics.

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