Read benchmarking in IO_demo

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

#216 aims to set up a benchmarking mode for the IO_demo app. This work is purely focussed on writing large amounts of data with XIOS, so this issue is focussed on extending the benchmarking mode for reading.

### Further details of the issue.

This issue will cover the addition of read functionality within the benchmark mode of the IO_demo app. This will entail:
- enabling the app to read data in this configuration
- creating a strategy to generate data for testing
- creating a pipeline for useful data from our benchmarking configurations