Fix conversion between shapely and cf for lines

[jsignell]

I started working on conversion between shapely and cf for polygons and in that work I realized that I had misunderstood the roles of part_node_count and node_count in #460.

My new understanding is mostly based on: "Example 7.16. Polygons with holes" in https://cfconventions.org/cf-conventions/cf-conventions.html#geometries

Even though that example is for polygons it shows clearly that the part_node_count is meant to denote the edges of the individual shapes and node_count is meant to denote the edges of multi-shapes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f6c8a1f) 85.81% compared to head (ba4a67f) 93.93%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #493      +/-   ##
==========================================
+ Coverage   85.81%   93.93%   +8.11%     
==========================================
  Files          13       12       -1     
  Lines        2284     1714     -570     
  Branches      173        0     -173     
==========================================
- Hits         1960     1610     -350     
+ Misses        294      104     -190     
+ Partials       30        0      -30     

Flag	Coverage Δ
mypy	?
unittests	93.93% <100.00%> (-0.02%)	⬇️

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[dcherian]

Thanks!