Add class methods (as properties) to collect tree list attributes at run time.

To improve the high-level API add class properties should be to facilitate retrieval of tree list attributes.

``` python
class FVS(object):
    ...
    @property
    def trees_num_recs(self):
        return self.fvslib.contrl_mod.itrn
    @property
    def trees_dbh(self):
        return self.fvslib.arrays_mod.dbh[:self.trees_num_recs+1]
```
