feat: Add orphan2d action to mesh-doctor

[bd713]

This PR adds a new mesh-doctor action to detect and handle orphaned 2D elements that are not faces of any 3D cells.

Usage

# Check for orphaned 2D cells (report only)
mesh-doctor orphan2d -i mesh.vtu

# Save orphaned cells to separate file
mesh-doctor orphan2d -i mesh.vtu --orphanOutput orphans.vtu

# Save clean mesh without orphaned cells
mesh-doctor orphan2d -i mesh.vtu --cleanOutput clean.vtu

[jafranc]

Great work on catching internal faces !!

Is it really possible tho in a full tetrahedral mesh ?
Would returing the transformed mesh to merge those faces into it a way to target ? as all would then break down to tetra (bad tetra tho)