Added base for ifc sverchok modifier

It's located in "Modifiers" section of the properties (the same way as IFC Array or IFC Stair).
It requires Sverchok addon to be enabled (it's lazy importing it when it's needed, so there is no new constant dependency).

What you can now do with it:
1) Add sverchok graphs to this modifier and store simple data from sverchok in IFC ("Update data from sverchok). The "output" node of the sverchok graph has to be type of "Viewer Draw" and it's name should start with "ifcoutput" (no case-sensitive).
2) Import/export json of sverchok graphs.

src/blenderbim/blenderbim/bim/module/model/prop.py

@@ -20,7 +20,7 @@ import bpy
 from blenderbim.bim.prop import ObjProperty
 from blenderbim.bim.module.model.data import AuthoringData
 from blenderbim.bim.module.model.root import ConstrTypeEntityNotFound
-from bpy.types import PropertyGroup
+from bpy.types import PropertyGroup, NodeTree
 
 
 def get_ifc_class(self, context):
@@ -294,3 +294,7 @@ class BIMStairProperties(PropertyGroup):
                 "tread_depth": self.tread_depth,
                 "tread_run": self.tread_run,
             }
+
+
+class BIMSverchokProperties(PropertyGroup):
+    node_group: bpy.props.PointerProperty(name="Node Group", type=NodeTree)