Add AbstractDIDResult with improved interface and subtypes

[junyuan-chen]

Several substantial changes/additions have been made:

1. A new abstract type AbstractDIDResult provides common interface for all subtypes used to collect results.
2. A new abstract type AggregatedDIDResult with related helper functions.
3. A concrete type SubDIDResult for viewing into a subset of a AbstractDIDResult.
4. Concrete types TransformedDIDResult and TransSubDIDResult for results obtained from linear transformation.
5. New functions lincom and rescale for linearly transforming coefficient estimates.
6. New functions apply, apply_and! and apply_and.
7. A new matrix type TableIndexedMatrix.

[codecov]

Codecov Report

Merging #20 (b36ee41) into master (888d6d4) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   99.75%   99.78%   +0.03%     
==========================================
  Files          10       10              
  Lines         816      942     +126     
==========================================
+ Hits          814      940     +126     
  Misses          2        2              

Impacted Files	Coverage Δ
src/DiffinDiffsBase.jl	100.00% <ø> (ø)
src/did.jl	100.00% <100.00%> (ø)
src/tables.jl	100.00% <100.00%> (ø)
src/utils.jl	100.00% <0.00%> (ø)

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