IfcMaterialLayerSet => IfcMaterialLayerList and other Set/List inconsistencies

Tiny, and maybe discussed before within MSG but I have witnessed people being ~~infuriated~~ slightly irritated about this:
Currently, we have

```
ENTITY IfcMaterialLayerSet
SUBTYPE OF (IfcMaterialDefinition);
MaterialLayers : LIST [1:?] OF IfcMaterialLayer;
LayerSetName : OPTIONAL IfcLabel;
Description : OPTIONAL IfcText;
DERIVE
TotalThickness : IfcLengthMeasure := IfcMlsTotalThickness(SELF);
END_ENTITY;
```
with **`LIST`** as a collection type (proper, because order of layers is very relevant) but **Set** in the naming

similar for
```
ENTITY IfcPolygonalFaceSet
Faces : LIST [1:?] OF IfcIndexedPolygonalFace;
```
I thought this was an inheritance from ISO 10303-42, but there we only have a (meaningful)
```
EBTITY connected_face_set
SUPERTYPE OF (OBEOF (closed_shell, open_shell))
SUBTYPE OF (topological_representation_item);
cfs_faces : SET [1:?) OF face;
END_ENTITY;
```

Begin with the IfcMaterialLayerSet and IfcPolygonalFaceSet declarations in the issue, then compare their LIST/Set semantics and names with the cited connected_face_set definition. Done means the project has an agreed resolution for these Set/List naming inconsistencies; no file or test is identified in the issue.

Written by the indexing model from the issue text.