Should `Categorical` lookups be ordered by default?

[tiemvanderdeure]

Categorical dimensions are assigned ForwardOrdered or ReverseOrdered if they are ordered, e.g.:

julia> da = rand(Dim{:var}(["temperature", "soil_moisture", "precipitation"]))
┌ 3-element DimArray{Float64, 1} ┐
├────────────────────────────────┴───────────────────────────────────────── dims ┐
  ↓ var Categorical{String} ["temperature", …, "precipitation"] ReverseOrdered
└────────────────────────────────────────────────────────────────────────────────┘
 "temperature"    0.148941
 "soil_moisture"  0.915144
 "precipitation"  0.576853

With #847 this might cause errors when indexing with a vector.

I would propose all Categorical lookups should be Unordered by default - there are very few cases where strings or symbols are really ordered (for Char you could argue otherwise though). There are many cases where a dimensions just isn't very big so the chance of it randomly being either ForwardOrdered or ReverseOrdered is quite big, and for dimensions of length 2 even 100%.