Add 2D honeycomb infill pattern

[elektrohenk]

Description

Like 2D lattice, this infill pattern is primarily intended for aircraft wings. Infill in wings keeps the thin skin from bending under aerodynamic loads, similar to ribs and stringers in classic wing structures. Secondary it keeps the skin from oil canning while printing. The spacing of the infill along the skin needs to be sufficiently low enough. 2D lines (aligned rectilinear) at the required spacing would be the lightest option. It does however not provide torsional stiffness (top and bottom layers will provide some stiffness but do not keep the skin from buckling under torsional load). For torsional stiffness closed cells are needed to create a torsion box. 2D lattice does provide the required stiffness, but 2D honeycomb should give the same results at 33% lower weight. In practice the infill lines need to be wider due to the 60 degrees overhang in a pure honeycomb. Therefore a infill overhang angle parameter was added that stretches the pattern vertically. At lower overhang angles the pattern keeps its weight advantage. 2D honeycomb has some disadvantages too. Its density is not evenly distributed vertically. The pattern can not be subdivided by itself which results in infill line disconnects when density is increased locally using modifiers.

Screenshots/Recordings/Graphs

Tests

• Checked many different infill parameter combinations.
• Checked added parameters in compare presets.
• Checked with modifiers.
• Printed several test parts.

Notes

• Special thanks to @LoftedAero for Add "2D Lattice" fill pattern for lightweight aircraft structures #8293 which was a very useful guide.
• I find the "2D" naming somewhat misleading since the (standard) honeycomb is also a 2D pattern but extruded along a different axis. It fits however nicely in the GUI.
• The icon shows the bed with a additional cyan line at the bottom to underline the different orientation.
• 2D isogrid/triangles was implemented experimentally for local reinforcement subdividing the 2D honeycomb pattern without disconnects, but it's density is not spread nicely for support of small details requiring extra increase of density which made it heavier and higher density 2D honeycomb was preferred (OrcaSlicer will anyhow disconnect the infill lines at horizontal modifier boundaries currently).