Finish notebooks

Complete notebooks in the following files:
- `examples/03_Existing Models.ipynb`
- `examples/05_Data Driven.ipynb`
- `examples/07_State Estimation.ipynb`
- `examples/08_Prediction.ipynb`

There are a few missing sections which are labelled with a note to be added in the next version of ProgPy. The goal of this effort is to create a complete book exploring ProgPy and prognostics.