refactor element_as_gdf internals

[jGaboardi]

This PR:

• starts towards #722
• resolves #723
• Should be reviewed and merged before #721

[codecov[bot]]

Codecov Report

Merging #724 (d32e1e1) into main (b1feadd) will decrease coverage by 0.1%. The diff coverage is 95.7%.

@@           Coverage Diff           @@
##            main    #724     +/-   ##
=======================================
- Coverage   99.4%   99.3%   -0.1%     
=======================================
  Files          4       4             
  Lines       1134    1140      +6     
=======================================
+ Hits        1127    1132      +5     
- Misses         7       8      +1     

Impacted Files	Coverage Δ
spaghetti/network.py	99.1% <92.3%> (-0.1%)	:arrow_down:
spaghetti/util.py	100.0% <100.0%> (ø)