Add checks for unititialized pyproj.crs objects to prevent core dumping

[snowman2]

• Closes pyproj.crs.PrimeMeridian() core dumps on Ubuntu #433
• Tests added
• Fully documented, including history.rst for all changes and api/*.rst for new API

[jorisvandenbossche]

Alternatively to doing those checks in all methods, can't we simply disallow creating an uninitialized object?

So that it already raises an error if you do d = Datum() ?

[snowman2]

We need to have that ability in the create methods:

pyproj/pyproj/_crs.pyx

Line 451 in aab1acb

cdef CoordinateSystem coord_system = CoordinateSystem()

So, this is the next best thing I could think of.

[jorisvandenbossche]

Looking for initialization in cython, I found in their docs:

... use a factory function instead that handles the object initialisation. It often helps to directly call __new__() in this function to bypass the call to the __init__() constructor.

https://cython.readthedocs.io/en/latest/src/userguide/special_methods.html

So that would also be an option I think

[snowman2]

Looking for initialization in cython, I found in their docs:

... use a factory function instead that handles the object initialisation. It often helps to directly call __new__() in this function to bypass the call to the __init__() constructor.

https://cython.readthedocs.io/en/latest/src/userguide/special_methods.html

So that would also be an option I think

That sounds like a much more secure/simple method for this. I will give it a go later.