allow dummy axes for relevant layers

[thomasp85]

Fix #440

This PR also removes the needs_stat_transform() trait method since it is no longer being used (all layers goes through a stat phase)

[teunbrand]

I'm sorry to keep pushing on this point again, but do you think we could make more use of the pre-existing infrastructure for this?

For example, I think the optional positions might have DefaultAestheticValue::String() with the dummy value. I'd be happy to have position literals materialised as proper columns somewhere at the beginning if that is the bottleneck. The precedent for this is process_annotation_layer also treating position literals similarly.

[thomasp85]

I'm happy you push back, but I'm not sure I see an upside to your proposal... If we use a string sentinel as default then wouldn't the user be able to inadvertently provide that in a plot and be surprised. To me, Null seems like the right choice

[teunbrand]

We can also expand the enum with DefaultParamValue::Dummy if strings are the problem. I'm not sure I understood the scenario correctly, you'd mean that the user would write MAPPING '__ggsql_dummy_' AS x and be surprised that the axis get surpressed?

[thomasp85]

yeah, basically... I think it is bad design to let this seep out however unlikely

I'm fine with adding an explicit dummy instead