scivision/pymap3d: Enable fallback to Python 2.6, 2.7 and 3.4

Full functionality of PyMap3D (as with Python in general in 2019) requires Python ≥ 3.5, but a useful subset is made available to Python 2.6, 2.7 and 3.4. This is relevant to MicroPython and ROS users and other special use cases where currently supported Python versions may not yet be readily available. Put Ellipsoid() in its own file for clarity and flexibility with other packages. ECI functions previously had an API unlike other PyMap3D functions, they are now the same as the rest of PyMap3D (scalar or N-D array for each variable)