TOO mode for OpSim

[soares-santos]

We suggest implementing a TOO mode into next OpSim runs. The example science case is Gravitational Wave observations triggered by LIGO.

[drphilmarshall]

Thanks @soares-santos ! How do you like the proposal that @ebellm submitted in #77? Feel free to close this our when you are happy with the markdown files.

[drphilmarshall]

Hooray!