Remove dependency on DataArray and DataFrame

[davidanthoff]

Fixes #45.

[codecov]

Codecov Report

Merging #47 into master will decrease coverage by 1.32%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage   90.11%   88.79%   -1.33%     
==========================================
  Files           1        1              
  Lines         172      116      -56     
==========================================
- Hits          155      103      -52     
+ Misses         17       13       -4

Impacted Files	Coverage Δ
src/ExcelReaders.jl	88.79% <100%> (-1.33%)	⬇️

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