Fail to import wgs84 separately.

This fails to execute:
>>> from navpy.core import wgs84
The only way to do this is to execute:
>>> from navpy.core.wgs84 import *
which clutters the namespace.

Fixed by removing "del wgs84" from the __init__.py. This makes navpy.wgs84
accessible from navpy, but this seems to be the common practice anyway (cf. scipy.linalg)