Add an example model that illustrates the use of the Adaptive Time Stepping (ATS) feature

[rbwinst-usgs]

Is your feature request related to a problem? Please describe.
I received a "bug report" related to MODFLOW 6 in which the user had incorrectly specified the input for the ATS input file. I first tried to explain the problem by quoting mf6io.pdf, but the user was still confused. I then tried to suggest the user look at one of the example models that used ATS. However, I couldn't find such an example. I created a very simple example and copied the TDIS and ATS files into an email to explain the user's issue. With that, the user was able to understand and fix the problem.

Describe the solution you'd like
Include an example model with the ATS input file

[emorway-usgs]

@rbwinst-usgs, If the user has the ability to run python scripts, you can point him or her to the autotest scripts (located in the .\autotest directory) with the following names:
-test_gwf_ats_lak01.py
-test_gwf_ats01.py
-test_gwf_ats02.py
-test_gwf_ats03.py

[rbwinst-usgs]

I believe the user was creating the files manually with a text editor.