Allow `theory ekos` without grids being present

[felixhekhorn]

add argument to parser.get_yaml_information to allow only to ask for the name, so we can run eko without the actual grids being present.

Edit:
You for sure need the grid for the operator card to be generated, but once that's generated the EKO could be created without access to the grid.
This will be beneficial e.g., when running in a cluster so you don't need to submit the grid with the job.

1. We are no sure this is still a problem, so please check before starting.
2. This issue and Allow to run explicitly a grid / fktable #168 are partially connected

[giacomomagni]

As from #159 we support the nnpdf commondata, we can add an option to read the kinematics value and the grids/eko names from there, instead of the grids.