Launcher code refactoring

[seanpearsonuk]

This is a follow-up to #2611.

1. The current structure of the code is:

def launch_fluent(...):
    <argument processing>
    launcher = create_launcher()
    return launcher()

So that each launcher class is directly usable, we should clear all out of launch_fluent() and instead have it done within the scope of each launcher instantiation wherever necessary. This would be done via shared code.

2. Identify and eliminate any conditions like if mode == launch_mode_xyz (except for the point in the code where the type of launcher is decided!). The aim is to handle any such conditions not through boolean logic but through polymorphism, which has been facilitated by the introduction of the polymorphic launcher classes. Some of those conditionals were still recently present in the code, but I don't know if they were all eliminated in the last round of changes. Please check by searching for uses of the relevant enum.