Welleng

- With the addition of the `Nodes` class in `connector` it's getting easier to set up optimization and Machine Learning.
- Added an experimental `torque_drag` module. It's not calibrated or verified but it's good enough for testing out some algos with.
- Closed 47
- Added 50 and an example using the new `connect_points` function in `connector`.
- Squashed some bugs.
- Stopped flake8 complaining (a little).
- Other stuff I can't remember.

With this major new release:
- Added the ISCWSA MWD Rev5 error model and updated the unit test to validate the model against the ISCWSA test data.
- The addition of the new model necessitated an update of the `error` and `iscwsa_mwd` modules.
- A `SurveyHeader` class has been added to better manage survey data and working to different azimuth references.
- The `magnetic-field-calculator` dependency has been added to provide default magnetic field data when none is provided in the survey: currently this needs to be run online so will need to modify this in the event that the code is run offline.
- Fixed some bugs with the `connector` module.

- Added unit tests for the error and clearance modules:
- Updated the error module which is now within 0.001% of the ISCWSA standard test data.
- The clearance model is now within 0.5% of the ISCWSA standard set of well paths test data.

- Starting to implement unit testing to maintain functional integrity of the `error` and `clearance` modules.
- Couple of miner updates to the `connector` and 'exchange' modules.
- Updated the license to a more permissive flavor.