iOS doesn't give developers the kind of detailed access to the wi-fi radio needed to create this kind of app. If it exists, it will only be for jailbroken devices.
I looked up Xirrus. If I found the app you are talking about, it is for Windows. It will not work in iOS. With few exceptions, you can not download and use apps from websites. If they do not specifically say they are for the iPhone, iPad, or other iOS devices, they will not work on your iPad. iOS apps that do work from websites are web apps, and with very few exceptions need an internet connection to work.
About all you can do with the iPad is walk around (slowly) looking at the signal icon at the top left. It's not great, but it will give you an idea of where the strongest and weakest areas are.
Sorry.