Choose model structure from command line

[peter-t-fox]

At the moment, there is support in the argument parser for selection of a model structure from the command line, however, this is not definitive, as it is possible that it can be overridden by the model specified in the parameters.ini file.

The code should be updated to:

• [x] Only allow selection of the model structure via the command line
• [x] Remove references to the model structure from the parameters file
• [x] Update the documentation to indicate how to select the model structure
• [x] Confirm that the command line arguments and choice of model structure is logged in the outputs
• [x] Add support for the TempModel (also known as Irish2) to the choices available

[github-actions[bot]]

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