Add CSV output support to some aggregators

[zix99]

Add CSV output support to various aggregators. Can output to file or stdout

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[codecov-commenter]

Codecov Report

Merging #94 (b343079) into master (ab79389) will decrease coverage by 0.32%. The diff coverage is 74.16%.

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@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
- Coverage   89.91%   89.60%   -0.32%     
==========================================
  Files         104      107       +3     
  Lines        5305     5425     +120     
==========================================
+ Hits         4770     4861      +91     
- Misses        410      429      +19     
- Partials      125      135      +10     

Impacted Files	Coverage Δ
cmd/bargraph.go	94.44% <40.00%> (-5.56%)	:arrow_down:
cmd/heatmap.go	93.33% <40.00%> (-3.14%)	:arrow_down:
cmd/histo.go	88.67% <40.00%> (-2.41%)	:arrow_down:
cmd/reduce.go	91.30% <40.00%> (-1.65%)	:arrow_down:
cmd/tabulate.go	96.89% <40.00%> (-2.30%)	:arrow_down:
pkg/csv/aggWriters.go	76.47% <76.47%> (ø)
pkg/csv/csvfile.go	85.18% <85.18%> (ø)
cmd/helpers/output.go	95.00% <100.00%> (+7.50%)	:arrow_up:
pkg/aggregation/accumulator.go	100.00% <100.00%> (ø)
pkg/multiterm/nullterm.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes