add new cf calendar 'tai'

[jswhit]

same as 'proleptic_gregorian', but only valid for dates after 1958-01-01.

exceptions are raised if dates before 1958 are requested or a base date before 1958-01-01 is used.

Tests added by @ChrisBarker-NOAA

[jswhit]

@ChrisBarker-NOAA could you take a look?

[ChrisBarker-NOAA]

This looks good to me -- thanks Jeff!

It could use some tests, though. maybe I'll find a bit of time to write a couple soon -- but don't wait for me.

[ChrisBarker-NOAA]

Hmm -- I'm adding some tests -- stay tuned for that.

But looking at the code more, should assert_valid_date check for < 1958 for tai?

though it doesn't seem to know which calendar it's using, so I guess not, but there should be a check somewhere.

and there is -- before assert_valid_date() is called :-)

[ChrisBarker-NOAA]

Looking, I can't tell for sure, but I think that a tai calendar could get a TZInfo attached to it, which should be illegal.

[ChrisBarker-NOAA]

Scratch that -- I see that the cftime.datetime class init doesn't take that keyword anyway.

[ChrisBarker-NOAA]

OK -- I did add tests -- see: #389

Once I was thinking about it, I figured why not?