Attribute zone_nodes is a sum of nodes in subzones too

[PabloCastellano]

Seen in emmdim/guifinetstudio@0d6fc24#diff-c07a5c9f693247996909462a19cf4387R131

+#     Manos: Pablo is wrong in this one, because the attribute total of the CNML contains also
+#     information from the subzones. Thus len(nodes) is not the same as CNML attirbute zone_nodes 

[emmdim]

I was wrong in this one. The only actual problem with len(nodes) is that it cannot be used directly as CNMLZone attribute, and can only be added after all parsing is done. If you still think it is a useful attribute I can make the commit.

Another issue though that rises here is that in some cases you prefer to calculate values that already exist in the cnml. This is a personal choice I guess but since the values exist in the CNML they should be used, in my opinion. Any extra information should be added in other structures. For example see the CNMLWrapper and GuifiZone in: https://github.com/emmdim/guifinetstudio/blob/master/test.py