Add field-container length method

[adtzlr]

and also allow fields to be updated without splitting at offsets.

This fixes #231 and fixes #232.

[codecov-commenter]

Codecov Report

Merging #233 (1e053c2) into main (92c01ec) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 1e053c2 differs from pull request most recent head 7b396fa. Consider uploading reports for the commit 7b396fa to get more accurate results

@@            Coverage Diff             @@
##             main     #233      +/-   ##
==========================================
+ Coverage   96.59%   96.61%   +0.01%     
==========================================
  Files          66       66              
  Lines        2760     2774      +14     
==========================================
+ Hits         2666     2680      +14     
  Misses         94       94              

Impacted Files	Coverage Δ
felupe/_field/_container.py	100.00% <100.00%> (ø)
felupe/tools/_newton.py	98.93% <100.00%> (-0.05%)	⬇️

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