Initial DataFrames merge

[lintool]

What does this Pull Request do?

Based on discussion in #190 and the broad support for data frames expressed at the Toronto Datathon, this is the initial merge of the data frames branch.

The functionality is limited, but it doesn't break anything existing, so merging should be fine.

How should this be tested?

I even wrote a new test case!

[codecov]

Codecov Report

Merging #210 into master will decrease coverage by 0.52%.
The diff coverage is 52.17%.

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
- Coverage   67.29%   66.76%   -0.53%     
==========================================
  Files          32       33       +1     
  Lines         636      659      +23     
  Branches      124      124              
==========================================
+ Hits          428      440      +12     
- Misses        167      178      +11     
  Partials       41       41

Impacted Files	Coverage Δ
...c/main/scala/io/archivesunleashed/df/package.scala	100% <100%> (ø)
src/main/scala/io/archivesunleashed/package.scala	65% <45%> (-6.67%)	⬇️

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