Openpile

- fixed bug in the kernel when applying base springs.
- clean up some part of the root directory
- benchmarked Cowden Clay soil model against literature from Byrne et al (2020).

- fixed bug in kernel that was amplifying the soil resistance and yielding unrealistic forces in the
pile.

- new method to retrieve the p-y mobilisation of springs in Results via the `.py_mobilization()`
- update to the connectivity plot `openpile.construct.Model.plot()` that adds the soil profile to the plot
if a soil profile is fed to the model.
- tz- and Qz-curves following the well-known API standards are now included in `openpile.utils`
- updates to the documentation
- API p-y curves now officially unit tested

- new Pile constructor `openpile.construct.Pile.create_tubular` creating a
circular and hollow steel pile of constant cross section.
- new properties for `openpile.construct.Pile`: `weight` and `volume`
- new `openpile.construct.Pile` method: `set_I()` to change the second moment of area of a given pile segment
- new `SoilProfile.plot()` method to visualize the soil profile
- API sand and API clay curves and models now accept `kind` instead of `Neq` arguments to differentiate between
static and cyclic curves
- create() methods in the construct module are now deprecated and should not be used anymore. Precisely, that is the
case for `openpile. construct.Pile.create()` and `openpile.construct.Model.create()`.

Added
- PISA sand and clay models (called Dunkirk_sand and Cowden_clay models)
- Rotational springs and base springs (shear and moment), see `utils` module
- New set of unit tests covering the `Construct` module, coverage is not 100%.

Notes
- first release of openpile with simple beam and simple winkler analysis with lateral springs

Added
- `Construct` module with Pile, SoilProfile, Layer, and Model objects
- `utils` module with py curves
- `Analysis`modile with `simple_beam_analysis()` and `simple_winkler_analysis()`
- `Result` class that provides the user with plotting and Pandas Dataframe overview of results.