Add an alternative way of implementing Tauchen's method to make_tauchen_ar1()

[QingyuanFang]

Qingyuan: I closed Pull requests #1409 and initiate this new one.

Please ensure your pull request adheres to the following guidelines:

• Tests for new functionality/models or Tests to reproduce the bug-fix in code.
• Updated documentation of features that add new functionality.
• Update CHANGELOG.md with major/minor changes.

[QingyuanFang]

@llorracc @mnwhite

Because there have been many changes to HARK since pull request #1409 was made (for instance, restructuring of "distribution.py" into a folder), I closed the original pull request and initiate this new one.

Many thanks.

[mnwhite]

@QingyuanFang Can you add a test that demonstrates this new functionality? See /HARK/tests/test_distribution.py, and just add a new class (i.e. class TestTauchenAR1(unittest.TestCase):) that runs it and verifies valid output?

[QingyuanFang]

@mnwhite
I tried to add the new class TestTauchenAR1(unittest.TestCase) following your instructions.
Many thanks.

[mnwhite]

Because of the new setup for /HARK/distributions/, I needed to add your function name to the init.py file. Tests running now, and I'll update the changelog after they finish. Thank you for writing that test so promptly!

[mnwhite]

I forgot to commit the changelog before merging, ugh.