Implements a command -st (--start_time) in the run command.
Example:
- autosubmit run a29z -st '14:30:00'
- autosubmit run a29z -st '2020-09-18 13:30:00'
In the first case, the date defaults to the current date: 2020-09-18 14:30:00'.
It should not accept dates in the past.
@mcastril @dbeltran Please review the positioning of the code block in the run_experiment
function. It seems like the best choice.