Refactoring tests

[BradReesWork]

Redoing the organization of the python tests.

Merging both SG and MG together and using tag to specify runs
organizing tests by graph function

MG Test File Renaming

• MG test files now end with "_mg" rather than starting with "test_mg". This will make it easier to see that there are test pairs, SG and MG, since a listing will now group files together.

mg pytest marker

• created a pytest marker called "mg" to identify MG test and allow CI/users to either include or skip
• pytest -m mg to run all mg tests
• pytest -m "not mg" to skip all MG tests

sg marker
test only SG code
pytest -m sg

Added markers for "sg" and "ci" for future use
Fixed a number of test issues

closes #1481

[acostadon]

just found the one double paste. Otherwise looked good to me.

[rlratzel]

Looks good, thanks for tackling this one. I have a few very minor feedback items.

Also, I think I prefer explicitly marking the tests like you did here, but FWIW for anyone else reading this, one can also apply a marker to an entire file of tests by using a pytestmark variable, like this.

[rlratzel]

/merge