Remove support for predictions without specified data

[leeper]

• [ ] Remove the ability to call prediction() without new data. People can use fitted() for that instead. The default will continue to return predictions for the data used to estimate the model via find_data()
• [ ] Remove methods where data/newdata is not allowed
• [ ] Update documentation

This is needed because of the complexities introduced by at and #17, which for non-linear models require two calls to predict() the second of which requires data (and, if at, that argument also).