Integrate real LHCb workflow commands (Pre/PostExecution)

[aldbr]

We have various LHCbDIRAC workflow modules that would need to be designed differently. Here is the main idea:

• LHCbDIRAC-specific code should go in Pre/PostExecution commands
• LHCb-specific code should mostly go in lb-prod-run (in this prototype we will create a wrapper around the existing lb-prod-run command as a PoC, and then the goal will be to integrate it to the command itself.

We are going to proceed one command at time probably. The process will consist in:

• disentangling parts of the code that should go in Pre/PostExecution commands, and the lb-prod-run wrapper
• once we all agree with the plan, proceed with the implementation
• integrate tests and documentation