combine: support Period for collapse

[iblislin]

• improve memory consumption and performance
• type promotion

close #498

[codecov-commenter]

Codecov Report

Merging #500 (d5c356e) into master (bb7eec3) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #500      +/-   ##
==========================================
+ Coverage   88.03%   88.06%   +0.03%     
==========================================
  Files          12       12              
  Lines         702      704       +2     
==========================================
+ Hits          618      620       +2     
  Misses         84       84              

Impacted Files	Coverage Δ
src/combine.jl	96.84% <100.00%> (+0.06%)	⬆️

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[iblislin]

@clouds56 could you try this PR out?

[iblislin]

I'm going to merge this PR if no objection