Break-up analytical function and test

[MaceKuailv]

It is time to look at the trajectory really carefully.

Also some fixes to other small issues.

[codecov]

Codecov Report

Patch coverage: 84.09% and project coverage change: -0.31 ⚠️

Comparison is base (427b255) 92.81% compared to head (93723bc) 92.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
- Coverage   92.81%   92.50%   -0.31%     
==========================================
  Files          11       11              
  Lines        2629     2656      +27     
  Branches      720      726       +6     
==========================================
+ Hits         2440     2457      +17     
- Misses         85       92       +7     
- Partials      104      107       +3     

Impacted Files	Coverage Δ
seaduck/lagrangian.py	93.52% <83.11%> (-2.18%)	⬇️
seaduck/oceinterp.py	93.24% <83.33%> (-0.88%)	⬇️
seaduck/eulerian.py	93.95% <100.00%> (ø)
seaduck/smart_read.py	95.23% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[MaceKuailv]

Changed dask to dask[array] as discussed near the end of issue #49

[MaceKuailv]

Too many bug unnoticed when developing on this branch. Abandoned in favor of #58