Further work on model.py template

Add __init__() call to base Model class.

Add note to argparse parser object that `verbose` option is not
implemented yet.

Add new `span` and `past` arguments to parser object to allow user to
initialise a model with a solution and historical period from the
command line. Add corresponding code in main scope to handle new
arguments.

Add new `define` and `set` arguments to parser object to allow user to
alter variables in the command-line call. Add corresponding code in
main scope to handle new arguments.

Add checks in main scope to only solve the model after successful
initialisation.