Filter and DropMissing

[eliascarv]

This PR add Filter and DropMissing transformations.

[codecov-commenter]

Codecov Report

Merging #28 (10e53da) into master (efd8914) will increase coverage by 0.48%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   92.00%   92.48%   +0.48%     
==========================================
  Files          14       15       +1     
  Lines         375      399      +24     
==========================================
+ Hits          345      369      +24     
  Misses         30       30              

Impacted Files	Coverage Δ
src/transforms.jl	97.82% <ø> (ø)
src/transforms/filter.jl	100.00% <100.00%> (ø)

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[juliohm]

As usual a great PR 👍🏽 we just need to double check the testmissing helper function, maybe we could avoid introducing a function if all we need is to check the positions of the missing values with findall.

[juliohm]

Also, we need to add these two new transforms to the README table 👍🏽

[eliascarv]

This PR is ready for a final review @juliohm!

[juliohm]

I think we are ready for merging. I will adjust the descriptions in the README.