Fix warning: src/cern/cpymad/libmadx.pxd:64:18:

Non-trivial type declarators in shared declaration (e.g. mix of pointers
and values). Each pointer declaration should be on its own line.
Return only basic data types in libmadx.pyx

This module should be only a light wrapper around the MAD-X C-API.
Convenience functions are the responsibility of the madx.py module.
Simplify numpy array creation in libmadx.pyx

Since we are not copying/freeing any data the ArrayWrapper code in
libmadx is unnecessary and only introduces a (minor) memory leak: the
allocated array objects themselves can't be freed due to the call to
Py_INCREF.

Explicitly documenting that libmadx.get_table is not responsible for
copying data resolves #45.