Improve developer experience for running binaries from test suite

[nathanjmcdougall]

I've added a bit of documentation about test dependencies to the README, and added a .gitignore entry for pypestworker log files that get generated.

Although perhaps, rather than adding autotest/pypestworker_*.txt to .gitignore, it would be better to reconfigure the workers to write the log files to autotest/temp - happy to take advice on that.

I tried building MODFLOW-USG from source without much luck - so I tried git checkout 96b16ef1544277a5cf64642b3aa5b5d2f89c4009 -- bin/win/mfusg.exe to get an old binary and renamed it to mfusg_gsi.exe ... happily it worked and I was able to pass autotest\pst_from_tests.py::usg_freyberg_test :)
It would be good if you could provide advice on this to improve/correct the information I've got in the README for this one.

Finally, there's a mention of mt3dusgs here:

pyemu/autotest/conftest.py

Lines 63 to 72 in 5dd3c78

	def full_exe_ref_dict():
	"""
	Get a dictionary of executable references for the project.
	"""
	d = {}
	for exe_name in [
	"mfnwt", "mt3dusgs", "mfusg_gsi", "mf6",
	"pestpp-ies", "pestpp-sen", "pestpp-opt", "pestpp-glm",
	"pestpp-mou", "pestpp-da", "pestpp-sqp", "pestpp-swp"
	]:

But I can't find mt3dusgs referenced anywhere else - maybe it can be removed?

[coveralls]

coverage: 77.899% (+0.03%) from 77.871%
when pulling c3febb3 on nathanjmcdougall:dev/improve-pestpp-test-dx
into 5dd3c78 on pypest:develop.

[briochh]

thanks @nathanjmcdougall,

pypestworker log files are default written to current directory so I think ok to gitignore for local tests.

for the modflow bins, the CI leans on https://github.com/modflowpy/install-modflow-action. For local tests maybe we should be pointing developers to the https://github.com/MODFLOW-ORG/executables/releases source. There is also a flopy get-modflow utility that users could leverage.

probably the mt3dusgs reference could be dropped -- I think the test suite is covered off by mf6 and some mfnwt