First needs to work with nested loops

[gordonwatts]

Fixes loops that occur after a .First() predicate. The below code now works.

We also normalized how we fetch the scope when accessing and filling the ntuple. This could introduce a bug, as it isn't clear how this should work in all contexts yet. Need a test to fail!

Fixes #225

pv_query = (
    ds
    .Select(lambda e: e.Vertices("PrimaryVertices"))
    .Select(lambda vtx: vtx.First())
)

track_query = (
    pv_query
    .Select(lambda v: v.trackParticleLinks().Where(lambda t: t.pt() > 500.0))
    .Select(lambda tracks: {
        "pt": tracks.Select(lambda t: t.pt()),
        "eta": tracks.Select(lambda t: t.eta()),
        "phi": tracks.Select(lambda t: t.phi()),
    })
)

tracks= track_query.AsAwkwardArray().value()

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.05%. Comparing base (7a7f9e4) to head (bd496c8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #226      +/-   ##
==========================================
- Coverage   95.05%   95.05%   -0.01%     
==========================================
  Files          35       35              
  Lines        2003     2002       -1     
  Branches      265      265              
==========================================
- Hits         1904     1903       -1     
  Misses         64       64              
  Partials       35       35              

Flag	Coverage Δ
unittests	95.05% <100.00%> (-0.01%)	⬇️

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