Default marker type for non-standard supports in 3D

[ccaprani]

Running into a crash in this section: https://github.com/sewkokot/opsvis/blob/04c12f7473b4c06e2d6825ae0fc267cf14b9036e/opsvis/model.py#L474 For grids (and other structures), there are often some supports that are arbitrary restraints of DOFs and not just "fixed" or "pinned". The code in here has no default for 3D structures for m_type that is for some support that is neither pin nor fixed and it crashes. Thanks!