Use directives to manipulate _tx_model_params

The easiest way to implement how the user can pass _tx_model_params to the main Python executable is via command line flags (status quo). Later on we can introduce "directives", i.e. instructions in the source code that are not part of the model. Similarly, we pass such instructions after the #! character sequence in shell scripts and after % in Jupyter notebooks.

Example

Let us consider an instance file _instance_file_ containing the instance UUID together with the launchpad data (serialized Launchpad object) in order to locate the program instance. Then, we have three possibilities:

1. Execution as a script program.vm passed as parameter to the interpreter script (status quo):

run_remotely.py [--instance=<_instance_file_>] program.vm

2. Execution as an executable script program.vm

#!/path/to/interpreter -i _instance_file_
...

./program.vm

3. In Jupyter using a so-called magic:

%instance: _instance_file_