Refactor generate_script function and put relevant parts in a header

[corentincarton]

Is your feature request related to a problem? Please describe.

The generate_script function should be refactored to be more modular: https://github.com/ecmwf/pyflow/blob/develop/pyflow/nodes.py#L1412

Describe the solution you'd like

A nice improvement could be to put all the header into an include to hide the redundant information from user (most of the header is the same for all tasks).

Describe alternatives you've considered

No response

Additional context

No response

Organisation

No response

[corentincarton]

We could do the following for instance:

#!/bin/bash

#TROIKA ....

# custom exit/cleanup code
exit_hook () {
    echo "cleaning up ...."
}

%include<trap.h>

export MYVAR=...
...

%nopp
...
%end

%include<exit.h>