make PRT shape tree width/depth limits configurable

[mistafunk]

complex rules and large input meshes (also multipolygon) can easily trip over the default shape tree width/depth limits.

these limits can be configured with attributes: https://esri.github.io/esri-cityengine-sdk/html/classprt_1_1_initial_shape_builder.html#a54112771964d72bdad2ce9d6a28fe20a

basically boils down to verify that attribute names are allowed to have forward slash characters