Add index of parameter set to command line arguments

[peter-t-fox]

When running in forward prediction mode, an index is specified in parameters.ini which represents the choice of parameter set to select from the posterior_parameters.csv file.

We want to move the index specification from the parameters file to the command line.

• [x] Add support to the argument parser for the index
• [x] Add error checking of the index value
• [x] Require that the index should be provided if running in prediction mode
• [x] Log the selected index in the outputs
• [x] Document the new functionality in the README

[github-actions[bot]]

Heads up @thibaud-porphyre @peter-t-fox - the "Covid19_EERAModel" label was applied to this issue.