Geo-espresso

Changed examples

- ReceiverFunctionInversion: added a 3-layer example (4th example)

New examples

- `SurfaceWaveTomography` examples

Changed examples

- Naming changes
- `FmWavefrontTracker` -> `FmmTomography`
- `XrayTracer` -> `XrayTomography`
- `ReceiverFunction` -> `ReceiverFunctionInversion`
- `GravityDensity` -> `GravityInversion`
- `GreatCircleTracing` -> `SurfaceWaveTomography`

Espresso core

- Template: Minor naming changes in templates
- Template: Return `matplotlib.axes.Axes` instead of `matplotlib.figures.Figure` in plotting methods

Changed examples

- Improve `ReceiverFunction.starting_model` and `ReceiverFunction.plot_model`

- Fix `ReceiverFunction.log_prior` by adding more constraints

Changed examples

- Improve performance of `ReceiverFunction.log_likelihood` by pre computing covariance matrix

Changed examples

- Fix `log_likelihood` in `ReceiverFunction`