support "replay" mode

for postprocessing it would be useful to have read in data in hdf5 files step by step and apply analysis routines to the saved data. For this it would be useful if there was some suport in the flesh and driver eg:

• set time and iteration based on what is found in the data file
• read grid structure from data
• create grid functions for data in files without having to activate the corresponding thorns (which might be an evolution thorn whose evolution cannot be disabled or that depends on very many other thorns)

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