IO_demo CRUN configurations

[EdHone]

Are there any linked Issues or Pull Requests?

No response

Brief description

Continuation runs (CRUNs) are a piece of essential I/O functionality and are currently un-tested within the IO_demo app.

Further details of the issue.

This issue will add the ability for the IO_demo app to run continuation runs, i.e. runs that write a checkpoint file and then restart from that same file. This needs to be tested for fields of each function space, so extra plumbing needs to be done to enable that within IO_demo.