Updated Writing ISIS3 Tests Using Gtest and Ctest (markdown)

# Test names

We use gtest as our unit testing framework, but use ctest to actually run the tests. The test name can be defined in the test definition. The naming convention for gtests is `UnitTest<Object><test case>` for unit tests and `FunctionalTest<app name><test case>` for app tests each in upper camel case. Source files should follow a similar pattern of `UnitTest<Object>.cpp` and `FunctionalTest<app>.cpp`. The documentation for how this works can be found in [cmake's documentation](https://cmake.org/cmake/help/v3.13/module/GoogleTest.html).

>**Note:** App names are considered a full word for camel case purposes. So when naming an test file or function, only capitalize the first letter of the app name. 

To run the gtests for a specific class, use `ctest -R ClassName`

To run all gtests, use `ctest -R "\." -E "(_app_|_unit_)"`. Not that this command simply excludes old Makefile based tests that contain the substrings `_app_` and `_unit_` in their respective names.