support BlockStatement in custom model

[karussell]

This PR makes it possible to nest statements to easier build sophisticated custom models.

The if and else_if statements have a then entry that allows a list of statements:

{ 
  "if": "country == DEU",
  "then": [
     { "if": "road_class == PRIMARY", "multiply_by": "0.8"  }
  ]
}

And else allows this list directly:

{ "if": "max_speed > 70", "limit": "70" },
{ "else": [
     { "if": "road_class == PRIMARY", "multiply_by": "0.8"  }
  ]
}

Further nesting is currently possible (maybe we should limit it to 2 levels?):

{ 
  "if": "country == DEU",
  "then": [
     { 
         "if": "road_class == PRIMARY",
         "then": [
            { "if": "max_speed > 70", "multiply_by": "0.5" }
         ]
     }
  ]
}

TODO

• adapt custom model editor