fix tick formatter with functions returning numerical values

[daschw]

Some recent changes introduced a bug when providing a function that returns numerical values to the axis formatter:

julia> plot(1:10, yformatter=y -> 1e3y)
Error showing value of type Plots.Plot{Plots.GRBackend}:
ERROR: MethodError: Cannot `convert` an object of type Float64 to an object of type String
Closest candidates are:
  convert(::Type{String}, ::String) at essentials.jl:218
  convert(::Type{T}, ::T) where T<:AbstractString at strings/basic.jl:231
  convert(::Type{T}, ::AbstractString) where T<:AbstractString at strings/basic.jl:232
  ...
Stacktrace:
   [1-8] ⋮ internal
       @ Base, Unknown
     [9] convert(#unused#::Type{Vector{String}}, a::Vector{Float64})
       @ Base ./array.jl:617
    [10] optimal_ticks_and_labels(ticks::Nothing, alims::Tuple{Float64, Float64}, scale::Symbol, formatter::Function)
       @ Plots ~/.julia/dev/Plots/src/axes.jl:199
    [11] get_ticks(::Symbol, ::Vector{Float64}, ::Vector{Any}, ::Tuple{Float64, Float64}, ::Vararg{Any})
       @ Plots ~/.julia/dev/Plots/src/axes.jl:301
    [12] get_ticks(sp::Plots.Subplot{Plots.GRBackend}, axis::Plots.Axis; update::Bool, formatter::Function)
       @ Plots ~/.julia/dev/Plots/src/axes.jl:222
    [13] get_ticks(sp::Plots.Subplot{Plots.GRBackend}, axis::Plots.Axis)
       @ Plots ~/.julia/dev/Plots/src/axes.jl:206

With this PR we get again

Furthermore, this PR improves (imho) the combination of log scale and latex formatter

plot(1:10, yscale=:log10, yformatter=:latex)

Before:

Now:

[codecov]

Codecov Report

Base: 90.65% // Head: 90.79% // Increases project coverage by +0.14% 🎉

Coverage data is based on head (7049aaf) compared to base (3fe21ae).
Patch coverage: 85.71% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4618      +/-   ##
==========================================
+ Coverage   90.65%   90.79%   +0.14%     
==========================================
  Files          41       41              
  Lines        8772     8779       +7     
==========================================
+ Hits         7952     7971      +19     
+ Misses        820      808      -12     

Impacted Files	Coverage Δ
src/axes.jl	89.87% <85.71%> (-0.09%)	⬇️
src/backends/gr.jl	91.66% <0.00%> (+<0.01%)	⬆️
src/utils.jl	92.65% <0.00%> (+0.16%)	⬆️
RecipesPipeline/src/utils.jl	97.26% <0.00%> (+1.36%)	⬆️
RecipesPipeline/src/series.jl	93.58% <0.00%> (+2.56%)	⬆️
src/examples.jl	99.20% <0.00%> (+3.17%)	⬆️
RecipesPipeline/src/RecipesPipeline.jl	100.00% <0.00%> (+11.11%)	⬆️

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