only show TTrees with the highest cycle number

[aminnj]

Since we don't provide a way to get a specific cycle number like f["Events;2"] we might as well hide all but the latest ttree. This assumes that TTree TKeys are in order of descending cycle number. We already use this assumption because f["Events"] goes to the first fName match. Can someone confirm this assumption is safe?

before

julia> UnROOT.samplefile("tree_cycles_hist.root")
ROOTFile with 6 entries and 23 streamers.
/Users/namin/.julia/dev/UnROOT/test/samples/tree_cycles_hist.root
├─ Events (TTree)
│  └─ "Jet_pt"
├─ Events (TTree)
│  └─ "Jet_pt"
├─ myTH1F (TH1F)
├─ myTH1D (TH1D)
├─ myTH2F (TH2F)
└─ myTH2D (TH2D)

after

julia> UnROOT.samplefile("tree_cycles_hist.root")
ROOTFile with 6 entries and 23 streamers.
/Users/namin/.julia/dev/UnROOT/test/samples/tree_cycles_hist.root
├─ Events (TTree)
│  └─ "Jet_pt"
├─ myTH1F (TH1F)
├─ myTH1D (TH1D)
├─ myTH2F (TH2F)
└─ myTH2D (TH2D)

[codecov]

Codecov Report

Merging #99 (011de72) into master (b8042cf) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
+ Coverage   87.14%   87.18%   +0.03%     
==========================================
  Files          11       11              
  Lines        1330     1334       +4     
==========================================
+ Hits         1159     1163       +4     
  Misses        171      171              

Impacted Files	Coverage Δ
src/displays.jl	97.95% <100.00%> (+0.18%)	⬆️
src/iteration.jl	78.10% <100.00%> (ø)

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