ISSUE #30

[JonSun42]

Modified the procedure of adding air spaces in the mesohyll of NeedleAnatomy

Air spaces are now placed (still post-fill) as rhombi in mid-wall positions between adjacent mesohyll cells:

Cell has new attribute protect_topology: bool = False if True wall flattening is inhibited. This is necessary to prevent rhombi's secondary axis to collapse. Resulting new vertices are promoted to junction nodes in CellGenerator

special_tissue.py has new method seat_air_spaces used in Organ and NeedleAnatomy

needle_mesohpyll_airspaces.py has been added as a diagnostic example (intentionally not put as a test to prevent computational effort for PRs)

Golden test has been modified in needle_default/needle_feature -> # air spaces
All other tests passed ✓

[JonSun42]

I will add network connections for new air spaces first.