Property concept wording

### Current text

- ./Property Sets/Property Sets for Contexts/README.md:The concept template _Property Sets for Contexts_ describes how an context can be related to a single or multiple property sets. A property set contains a single or multiple properties. The data types of an individual property are single value, enumerated value, bounded value, table value, reference value, list value, and combination of property occurrences.
- ./Property Sets/Property Sets for Materials/README.md:The concept template _Property Sets for Contexts_ describes how an context can be related to a single or multiple property sets. A property set contains a single or multiple properties. The data types of an individual property are single value, enumerated value, bounded value, table value, reference value, list value, and combination of property occurrences.
- ./Property Sets/Property Sets for Objects/README.md:The concept template _Property Sets for Objects_ describes how an object occurrence can be related to a single or multiple property sets. A property set contains a single or multiple properties. The data types of an individual property are single value, enumerated value, bounded value, table value, reference value, list value, and combination of property occurrences.
- ./Property Sets/Property Sets for Profiles/README.md:The concept template _Property Sets for Contexts_ describes how an context can be related to a single or multiple property sets. A property set contains a single or multiple properties. The data types of an individual property are single value, enumerated value, bounded value, table value, reference value, list value, and combination of property occurrences.
- ./Property Sets/Property Sets for Types/README.md:The concept template _Property Sets for Objects_ describes how an object type can be related to a single or multiple property sets. A property set contains a single or multiple properties. The data types of an individual property are single value, enumerated value, bounded value, table value, reference value, list value, and combination of property occurrences.

### The wording for all property concepts is consistent, good!, but I find it could use some improvement:

1. "The **data types** of an individual property"

Actually. Properties have two types. In the property set listing we generally refer to them as the Property Type and the Data Type. We could debate the naming for this but I don't think we should do that now.

In the template I would propose to articulate these two types though

2. "reference value, list value, **and combination of property occurrences.**"

First of all, "combinations" or "a combination" but more importantly I don't understand this. If I omit the fragments in between it reads:

> The data types of an individual property are a combination of property occurrences

Doesn't make sense to me. Is it referring to ComplexProperty?

### I propose (as a rough draft):

(edited with suggestions)

The concept template _Property Sets for XXX_ describes how an XXX can be related to one or more property sets. A property set (IfcPropertySet/IfcMaterialProperties/IfcProfileProperties) is a grouping of one or more properties (subtype of IfcProperty). The IfcProperty stores its name as a string (IfcIdentifier) in the model which differentiates it from schema attributes where the attribute name is provided as part of the schema. As such, it is used to store extensible data.

The individual subtypes of IfcProperty can describe single values (IfcPropertySingleValue), enumerated values (IfcPropertyEnumeratedValue), bounded values (IfcPropertyBoundedValue), table values (IfcPropertyTableValue), reference values (IfcPropertyReferenceValue) and list values (IfcPropertyListValue). The data type of such a property is always a member of the IfcValue select.

There are predefined property sets (now deprecated) and property sets provided as part of the IFC specification encoded in XML. Property sets are also the designated mechanism to store user-defined, vendor- and project-specific property sets.

~Should we make a reference to bSDD and/or IDS?~