Add section on inappropriate intimacy between classes

[ecomodeller]

Classes should interact through public APIs, not by reaching into each other's private attributes.

Example from MIKE IO plotting refactor:

Before (inappropriate intimacy):

# Plotter reaches into geometry internals for subsetting
values = values[self.da.geometry.top_elements]
geometry = self.da.geometry.geometry2d

# Plotter uses private attribute to check for time axis
if self.da._has_time_axis:
    return self.da.values[0]

After (using public API):

# Use sel/isel for subsetting
da = da.sel(layers="top")
geometry = da.geometry

# Use public dims property
da = self.da.isel(time=0) if "time" in self.da.dims else self.da

Key principles:

• Never access private attributes (prefixed with _) of another class
• Use public methods like sel, isel, and public properties like dims
• If you need to access private state, that's a signal the class is missing a public API
• Plotters should plot, not subset — keep data manipulation in the data layer