Better document floating point indexing

[Tokazama]

This came up as part of #35.

You might consider revising some of the documentation, though. I haven't seen anything wrong, but it doesn't reinforce the fact that indexing by float will be error-prone. For example, the Axis documentation includes some indexing by equality to a float. If you change time1 to be time1 = Axis((1.5:.1:10)s), then time1[1.7s] errors, as does time1[==(1.7s)], both of which are nearly the same as the current examples using 1.5s.

https://tokazama.github.io/AxisIndices.jl/stable/axis/

[Tokazama]

Hey @grahamas, master now has axis[closest(val)] which finds the closest key in axis to val for indexing. Let me know your thoughts and if this is helpful.

[grahamas]

Hey @Tokazama , sorry I didn't see this. This seems like a great solution, but it doesn't work in a case I'd expect:

A = AxisArray(3:6, 0.3:0.1:0.6)
A[closest(0.6)]

edit: though it looks like this is just a bug? It also doesn't work for Axis objects.

AxisIndices.Axis(0.3:0.1:0.6)[0.6 |> closest]
ERROR: MethodError: no method matching one(::Nothing)
Closest candidates are:
  one(::Union{Type{T}, T}) where T<:AbstractString at strings/basic.jl:262
  one(::Union{Type{P}, P}) where P<:Dates.Period at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Dates/src/periods.jl:54
  one(::NamedDims.NamedDimsArray{L, T, N, A} where {T, N, A<:AbstractArray{T, N}}, ::Any...) where L at /home/graham/.julia/packages/NamedDims/OBqS7/src/functions.jl:99
  ...
Stacktrace:
 [1] oneunit(x::Nothing)
   @ Base ./number.jl:318
 [2] _closest_number_forward(collection::StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}}, x::Float64)
   @ AxisIndices ~/.julia/packages/AxisIndices/Eb16j/src/closest.jl:31
 [3] closest_number
   @ ~/.julia/packages/AxisIndices/Eb16j/src/closest.jl:18 [inlined]
 [4] closest
   @ ~/.julia/packages/AxisIndices/Eb16j/src/closest.jl:10 [inlined]
 [5] Fix2
   @ ./operators.jl:1002 [inlined]
 [6] to_index(#unused#::AxisIndices.IndexAxis, axis::AxisIndices.Axis{Float64, Int64, StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}}, SimpleAxis{Int64, ArrayInterface.OptionallyStaticUnitRange{ArrayInterface.StaticInt{1}, Int64}}}, arg::Base.Fix2{typeof(closest), Float64})
   @ AxisIndices ~/.julia/packages/AxisIndices/Eb16j/src/closest.jl:69
 [7] to_index
   @ ~/.julia/packages/ArrayInterface/hIHGL/src/indexing.jl:283 [inlined]
 [8] getindex(axis::AxisIndices.Axis{Float64, Int64, StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}}, SimpleAxis{Int64, ArrayInterface.OptionallyStaticUnitRange{ArrayInterface.StaticInt{1}, Int64}}}, arg::Function)
   @ AxisIndices ~/.julia/packages/AxisIndices/Eb16j/src/getindex.jl:15
 [9] top-level scope
   @ REPL[7]:1