Add ifccityjson to start with supporting conversions between IFC and CityJSON files

src/ifccityjson/README.md

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+# CJ2IFC
+Converter for CityJSON files to IFC.
+
+-- WARNING --
+
+CJ2IFC only came into being 14/04/2021. Be prepared for lots of bugs, unfinished implementations and little to no documentation!
+
+## Dependencies
+- [IfcOpenShell](https://github.com/IfcOpenShell/IfcOpenShell)
+- [CJIO](https://github.com/cityjson/cjio)
+
+## Usage of CJ2IFC
+Following command will execute a conversion from CityJSON to IFC
+  
+    python CJ2IFC.py [-i input file] [-o output file] [-n name of identification attribute]
+
+The example file that could be used is example/3D_BAG_example.json
+
+    python CJ2IFC.py -i example/3DBAG_example.json -o example/3DBAG_example.ifc -n identificatie
+
+## Implemented geometries
+- [ ] "MultiPoint"
+- [ ] "MultiLineString"
+- [ ] "MultiSurface"
+- [ ] "CompositeSurface"
+- [x] "Solid"
+- [ ] "MultiSolid": exterior shell
+- [ ] "MultiSolid": interior shell
+- [ ] "CompositeSolid"
+- [ ] "GeometryInstance" 
+
+## TODO
+- [x] CityJSON Attributes as IFC properties in 'CityJSON_attributes' pset
+- [x] Implement georeferencing
+- [ ] Do not use template IFC for new IFC file, but make IFC file from scratch