add obsdim argument to obsview

[CarloLucibello]

This is a comeback of the ancient obsdim argument, in a simplified fashion.

In this new reincarnation, we don't require all functions in the library to deal with an obsdim argument, e.g. numobs and getobs have no knowledge of obsdim.

If users want to work with a non-standard (i.e. non-last) observation dimension for arrays, they wrap their data with obsview(data, obsdim) at the beginning of the pipeline, and then they forget about obsdim in the following transformations. This has been previously discussed here.

Since the slidingwindow method could be dealing often with non-standard obsdim (time), I add the argument also there. Close #199

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 42.85714% with 16 lines in your changes missing coverage. Please review.

Project coverage is 84.93%. Comparing base (e705227) to head (c1abdab).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/obsview.jl	60.00%	8 Missing ⚠️
src/slidingwindow.jl	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #202      +/-   ##
==========================================
- Coverage   86.05%   84.93%   -1.13%     
==========================================
  Files          14       15       +1     
  Lines         710      697      -13     
==========================================
- Hits          611      592      -19     
- Misses         99      105       +6     

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