Fix solve_troposphere bug

[betsy-loaiza]

The --solve_troposphere argument is defined with nargs=1, so when it is provided on the command line, argparse returns a list instead of an integer. However, the code later expects an integer value.

This causes an issue when the --solve_troposphere option is provided on the command line. The default value is not affected because it is stored as an integer.

This change adds a check to handle the case where solve_troposphere is a list by using its first element.

[demiangomez]

Actually, we should not test for list and drop the nargs=1 in the parser.add_argument. That way the parameter will always be an int. Then also remove the solve_troposphere = solve_troposphere[0].