Add implementation using Numpy C UFunc API

[mdboom]

For now, the tests from #1 can be used with this extension module -- but I think eventually we want to centralize the test suite and have it run against all of the implementations as a fixture.

Just to compare and contrast with #1: By using the UFunc API, this lets Numpy handle the broadcasting for us upfront, rather than handling it ourselves. I think the per-erfa-function lines-of-code is probably a little lower for this approach as well (though there is some extra helper functions at the top, we only have to get that right once).

[astrofrog]

Nice! I agree that we should centralize the test suite and also probably do some speed benchmarks.

[astrofrog]

Minor: I think this is missing the numpy include path?

[astrofrog]

@mdboom - could you show a usage example? At the moment:

In [21]: eraD2dtf('UTC',2,3.,4.)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-21-fc54acac0e60> in <module>()
----> 1 eraD2dtf('UTC',2,3.,4.)

TypeError: Not implemented for this type

Am I using it wrong?

[mdboom]

@astrofrog: I see that too. I did this kind of hastily, and I think I may have messed something up... Stay tuned...

[mdboom]

I'm running into the same problem here -- there doesn't seem to be a way to have ufuncs over string fields. Looking at the code, it seems there used to be a way but it got sort of papered over over time 😦

As discussed in #1, though, that parameter doesn't need to be vectorized, so we can wrap this in another function that just takes it as a scalar. That's where things start to get a little hairy, of course.

[mdboom]

I think #1 is already to be considered generally superior to this. It seems the UFunc API just isn't flexible enough for our needs -- I was hoping it would be a win since it handles the broadcasting etc. for us.

It may make sense to do a C implementation of #1 for comparison at some point, but I think I've convinced myself that the UFunc API is insufficient.