Add generic fallbacks for getobs and numobs

[darsnack]

Adds fallbacks as getindex and length for getobs and numobs.

[codecov-commenter]

Codecov Report

Merging #56 (aad5c72) into main (791fb77) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   88.86%   88.91%   +0.05%     
==========================================
  Files          13       13              
  Lines         413      415       +2     
==========================================
+ Hits          367      369       +2     
  Misses         46       46              

Impacted Files	Coverage Δ
src/observation.jl	85.41% <100.00%> (+0.63%)	⬆️

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[CarloLucibello]

we should document these fallbacks in the docstrings