Add ScaledArray, RotatingTimeValue and settime

[junyuan-chen]

1. The new array type ScaledArray is helpful for working with time data that are not integers. Otherwise, it may not be clear which value represents the next time period.
2. To allow working with rotating panels where the time periods may overlap across different rotation groups, RotatingTimeValue replaces the default integer reference values in ScaledArray. It can also be used as array index and supports some basic arithmetics.
3. The new function settime is only necessary when the time data are not integers.

[codecov]

Codecov Report

Merging #23 (f58d31d) into master (bba59a4) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
+ Coverage   99.81%   99.83%   +0.02%     
==========================================
  Files          10       11       +1     
  Lines        1053     1184     +131     
==========================================
+ Hits         1051     1182     +131     
  Misses          2        2              

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

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