Block: Allow any AbstractDateTime

[jpsamaroo]

Closes #34

[jpsamaroo]

I plan to add some tests using an ad-hoc PreciseDateTime that has nanosecond resolution.

[codecov]

Codecov Report

Merging #35 (0f07bc1) into main (614e79b) will decrease coverage by 0.12%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
- Coverage   92.43%   92.30%   -0.13%     
==========================================
  Files          20       20              
  Lines        1362     1326      -36     
==========================================
- Hits         1259     1224      -35     
+ Misses        103      102       -1     

Impacted Files	Coverage Δ
src/block.jl	90.36% <66.66%> (-1.31%)	⬇️
src/identity_map.jl	87.50% <0.00%> (-0.31%)	⬇️
src/graph.jl	95.12% <0.00%> (-0.23%)	⬇️
src/ops/sources.jl	92.75% <0.00%> (-0.21%)	⬇️
src/alignment.jl	92.91% <0.00%> (-0.14%)	⬇️
src/ops/align.jl	100.00% <0.00%> (ø)
src/evaluation.jl	100.00% <0.00%> (ø)
src/ops/lagging.jl	87.87% <0.00%> (+2.58%)	⬆️

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