Clarify roles and structure of Grid, Core and Data objects and use in models and axis validation.

[davidorme]

Is your feature request related to a problem? Please describe.

The current implementation of the core axis and axis validation passes in the Grid object and then kwargs. This was only ever really a placeholder for giving the axis validation setup access to more information from the configuration object to the axis definitions. More broadly, the Grid object is rather arbitrarily stuffed into Data and is passed around hidden in that.

The aim of this PR is therefore have consistent use of the three high level objects (Data, Config, Grid) in signatures. The Axis validators can then accept (Config, Grid) arguments and models can expect to use (Data, Config, Grid). We could shove Grid into Config - it is a much more logical home than Data - but from a quick chat we think having three objects with clear roles is easier to understand.

Describe the solution you'd like

We want the whole of the configuration and the grid to be passed in to axis validators. That should give access to all of the information required to configure the model and hence everything that an axis validator could want. We also need to update model code to accept Data, Config and Grid objects.

[davidorme]

@jacobcook1995 @TaranRallings @vgro - Does that seem reasonable as a restructure?