Make tick finding more compatible with Float64

[ffreyer]

While working on MakieOrg/Makie.jl#3663 I noticed that ticks are broken for 1e9 .+ (1:10):

In this case PlotUtils switches to fallback_ticks as 9 / 1e9 < sqrt(eps(Float64)) ~ 1e-8 and we get (min, 0.5 * (max + min), max). Our pretty-string functions then round those values to integers, I assume.

To avoid the PlotUtils part of the problem I think just adjusting the rtol is enough. With rtol = 1000.0 * eps(T) the tolerance should be about the same for Float32 and 1e-13 for Float64. With that the plot becomes:

[asinghvi17]

From the test logs it looks like this fails when the input is an Int64, since there's no definition of eps for that.

[t-bltg]

Could you also add some tests to runtests.jl ?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.83%. Comparing base (f16e07a) to head (e9a0491).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
- Coverage   72.05%   71.83%   -0.22%     
==========================================
  Files           7        7              
  Lines         662      664       +2     
==========================================
  Hits          477      477              
- Misses        185      187       +2     

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[asinghvi17]

Looks good to me, thanks @ffreyer!