Imputor TestSuites

[rofinn]

• Use TestSuites to avoid mostly duplicated tests for each Imputor type (closes #46)
• Fixes some error conditions with completely missing datasets (closes #45)
• Fixes ambiguity errors when using rowtables
• Fixes deprecation of GroupedDataFrame broadcasting on 0.19.2

TODO:

• [x] Cleanup existing tests that duplicate behaviour from the testsuites
• [ ] Test chains in the testsuite
• [x] Add tests for dataframe groupby

[codecov[bot]]

Codecov Report

Merging #47 into master will decrease coverage by 0.1%. The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage   96.68%   96.58%   -0.11%     
==========================================
  Files          11       11              
  Lines         211      234      +23     
==========================================
+ Hits          204      226      +22     
- Misses          7        8       +1

Impacted Files	Coverage Δ
src/imputors/interp.jl	100% <100%> (ø)	:arrow_up:
src/imputors/srs.jl	100% <100%> (ø)	:arrow_up:
src/imputors/locf.jl	100% <100%> (ø)	:arrow_up:
src/imputors/nocb.jl	100% <100%> (ø)	:arrow_up:
src/imputors/fill.jl	100% <100%> (ø)	:arrow_up:
src/imputors.jl	100% <100%> (ø)	:arrow_up:
src/imputors/drop.jl	96.87% <66.66%> (-3.13%)	:arrow_down:

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[rofinn]

Thanks for taking a look over it before you leave.

[rofinn]

@oxinabox Do you have any more comments? I'd like to get this in before I do an more refactoring.