Updated Writing ISIS3 Tests Using Gtest and Ctest (markdown)

## Creating a basic callable function

For the rest of this document we will use `appname` as the name of the application that is being worked on.

For the rest of this document we will use `appname` as the name of the application that is being worked on. Simple example at https://github.com/USGS-Astrogeology/ISIS3/tree/dev/isis/src/base/apps/crop 

1. In the `appname` folder create two new files, `appname.cpp` and `appname.h`. These files are where the application logic will live.

1. In `appname.h` and `appname.cpp` create a new function in the `Isis` namespace with the following signature `void appname(UserInterface &ui)`.

1. Copy the contents of `IsisMain` in `main.cpp` into the new function.

1. In `appname.h` and `appname.cpp` create a new function in the `Isis` namespace with the following signature `void appname(UserInterface &ui)`. If the application has a `from` cube, add a second function with the input cube `void appname(Cube incube, UserInterface &ui)`. 

1. Copy the contents of `IsisMain` in `main.cpp` into the new `void appname(Cube incube, UserInterface &ui)` function. Rewrite `void appname(UserInterface &ui)` to unpack the "from" cube into `void appname(Cube incube, UserInterface &ui)` (if the app doesn't take an input cube, simple copy the contents into` void appname(Cube incube, UserInterface &ui)`)

1. Copy any helper functions or global variables from `main.cpp` into `appname.cpp`. So as to not pollute the `Isis` namespace and avoid redefining symbols, forward declare any helper function in `appname.cpp` and do not define them in `appname.h`. 

1. Put all of the required includes in `appname.cpp` and `appname.h`.

1. Remove the call to get the UserInterface; it usually looks like `UserInterface &ui = Application::GetUserInterface();`.