Generating lpy code fails if default catergory name is set to another name

This ..

```py
lp = LsystemParameters()
with io.open('my.json') as file:
    lp.load(file)
lp.set_defaut_category('my')
lp.generate_py_code()
```

fails with `default_category` not being a property of `self`.

https://github.com/fredboudon/lpy/blob/master/src/openalea/lpy/lsysparameters.py#L500-L508

