fix generation of class contructors

This needs review.

Every use of desc['name'] in xdress/cython.py is probably suspect.

Fixes errors such as the following
```
Error compiling Cython file:
------------------------------------------------------------
...


    def __init__(self):
        """Untemplated(self)
        """
        self._inst = new cpp_untemplated.Untemplated()
                        ^
------------------------------------------------------------

Error compiling Cython file:
------------------------------------------------------------
...


    def __init__(self):
        """Untemplated(self)
        """
        self._inst = new cpp_untemplated.{'tarbase': 'basics', 'tarname': 'Untemplated', 'language': 'c++', 'srcname': 'Untemplated', 'sidecars': ('src/basics.py',), 'incfiles': ('basics.h',), 'srcfiles': ('src/basics.cpp', 'src/basics.h')}()
                                        ^
------------------------------------------------------------
```