Use consistently primary/secondary to refer to grids and data dictionaries

[jhabriel]

In many places, there is extended use of g_h and g_high to refer to the higher-dimensional grid (same for d_h and d_high) and g_l and g_low to refer to the lower-dimensional grid (same for d_l and d_low). Ideally, we should homogenize these references using g_primary (d_primary) and g_secondary (d_secondary), respectively.

[keileg]

I will try to help out. See my comments in #477, though: We can afford some pragmatism in how aggresive we are in rooting out the old terminology.

[keileg]

This should to a large degree have been solved in #673, but we'll keep it open for a while as a reminder.