Improve ScaledArray and add RotatingTimeRange

[junyuan-chen]

1. ScaledArray now supports DataAPI.invrefpool.
2. Changes are made for future support of ScaledArray with Missing values, though it is not completed yet.
3. The new type RotatingTimeRange allows the elements of ScaledArray to be RotatingTimeValues.

[codecov]

Codecov Report

Merging #24 (af4abe6) into master (fc03843) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #24   +/-   ##
=======================================
  Coverage   99.83%   99.84%           
=======================================
  Files          11       12    +1     
  Lines        1184     1253   +69     
=======================================
+ Hits         1182     1251   +69     
  Misses          2        2           

Impacted Files	Coverage Δ
src/DiffinDiffsBase.jl	100.00% <ø> (ø)
src/utils.jl	100.00% <ø> (ø)
src/ScaledArrays.jl	100.00% <100.00%> (ø)
src/operations.jl	100.00% <100.00%> (ø)
src/procedures.jl	100.00% <100.00%> (ø)
src/time.jl	100.00% <100.00%> (ø)

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