Class definition in the IFC schema

In the current examples, we (re)define the class object for every classification, and it always has a code and a URI. The core schema (ifcx?) could have a class definition already in there. The advantage of that approach would be easier UI support, because every class would have the same attributes.

CURRENTLY:
```
"schemas": {
"EXAMPLE::class": {
"value": {
"dataType": "Object",
"objectRestrictions": {
"values": {
"code": {
"dataType": "String"
},
"uri": {
"dataType": "String"
}
}
}
}
}
}
```
PROPOSED:
```
"schemas": {
"ifcx::class": {
"value": {
"dataType": "Object",
"objectRestrictions": {
"values": {
"system": {
"dataType": "String"
},
"code": {
"dataType": "String"
},
"uri": {
"dataType": "String"
}
}
}
}
}
},
"data": {
"ifcx::class:"{
"system": "EXAMPLE",
"code": "ABC-001",
"uri": "https://identifier....."
}
}
```

Related to the schema wishlist #51 - perhaps this could be also resolved with the 'extends':

```
model EXAMPLE::class extends ifcx::class { }
```