Improve setting of is_manager and nworkers in ensemble class

[shuds13]

When parse_args is not used, ensemble class does not set nworkers or is_manager.

This is somewhat non-trivial due to the different order in which things can be set. However, it is particularly problematic that if ensemble.is_manager does not work at the end of a script.

I have looked at setting these in the libE_specs setter function when they are set in new_specs, but suprisingly, this seems to contain default values in addition to those explicitly specified. I don't want to assume a default MPI here if it is not set, so maybe we need to duplicate what we do in the validators in default_comms.

[shuds13]

Fixed by #1336