Automatic installation

[alefeve]

We need to be able to install the library by just down loading the code from this repo and run the build and install script.
Currently, the install script needs to be edited to set the following variables:

• RV_HOME
• RV_PYTHON_EXE
• RV_PKG_EXE
• RV_PKG_PATH
• RV_PYTHON_PATH
• RPA_WHL
• RPA_CORE_PKG
• RPA_WIDGETS_PKG

In our rez build script, we want to set these variables as we need, then download the code, and run the build and install scripts.
So we would need the install script to not set them to some default values, and just raise an error if they're not set.