ENH: add 'asm8' to NaT

[llllllllll]

I was suprised to find that NaT.asm8 does not exist though there is a valid nat form for numpy.datetime64.

This also adds tests for the asm8 behavior for a random sample of times and NaT

[jreback]

why is this needed?

[jreback]

ahh sorry I see u moved to _Timestamp

[jreback]

lgtm, ping when green.

[llllllllll]

Thank you!

One question I have about the NaT class heirarchy is the purpose for the _NaT class. It is only sublclassed by NatType and seems to add a few operations.

[jreback]

the _NaT is a sub-class of _Timestamp class which is a cdef class so its purely a c-extension type. While NaTType and Timestamp are available to the python space (and the c-space).

The entire reason for this is that datetime is untimately sub-classes in c-land as its a python extension type.

@wesm original comment.

# This is PITA. Because we inherit from datetime, which has very specific
# construction requirements, we need to do object instantiation in python
# (see Timestamp class above). This will serve as a C extension type that
# shadows the python class, where we do any heavy lifting.

[llllllllll]

I see, it appears cython doesn't give us a way to override the tp_new directly so we cannot create the overridden __new___ with the correct signature. Thanks for explaining this.

[llllllllll]

We are green!

[jreback]

thank you sir!