Welly

- Load your well in feet! The number one most hated 'feature' has been 'fixed'... you can now pass the `index` argument to `Well.from_las()` or `Well.from_lasio()` to control how the index is interpreted. Use `'existing'` or `'original'` to keep whatever is specified in the LAS file (probably what you want). To convert to metres, use `'m'`; to convert to feet use `'ft'`.
- In the next point release, v0.5, we will change the default behaviour to `'original'`, so if you want to keep forcing to metres, you'll have to change your code to `Well.from_las(fname, index='m')`. There is a `FutureWarning` about this.
- The `Curve` object now has a `basis_units` attribute carrying this information. Either `'m'` or `'ft'`.
- See `tutorial/Well_depth_units_v0.4.7.ipynb`.
- Thank you to Kent Inverarity for implementing this long-hoped-for feature.

- Big fix in `Location`.

- Allowed adding the NULL value when writing an LAS file with `w.to_las()`.

- Dropped support for Python 2.7 and Python 3.4, and added support for Python 3.7 and 3.8.
- Fixed `location`, whose changes were inadvertently rolled back.

- You can now pass an `alias` dictionary to `Well.df()`, along with the list of `keys`. You can pass `keys` and `alias` to `Project.df()` as well.
- A new function, `location.trajectory()`, generates an evenly sampled trajectory, given a sample spacing in metres.
- Added `location.plot_plan()` and `location.plot_3d()` for plotting well trajectories.
- Added a new tutorial notebook, `tutorials/Location.ipynb` to demonstrate the well path capabilities of `Well.location()`. The notebook does not cover geographic CRS's. There's still a short example in `Well.ipynb`.
- Fixed some buggy behaviour when creating 'empty' wells, and added example to top of `tutorials/Well.ipynb`.
- You can now pass a URL directly to `Well.from_las()` and it will try to read it.

- Implemented basis updating when slicing. In general, you probably want to 'slice' (get a subcurve) using `curve.to_basis()` because you can use depth to get the section you want. But if you want to use indexing, like `curve[100:110]`, this operation now updates `curve.start` so that `curve.basis` is therefore updated.
- Breaking change: `utils.top_and_tail` now only works on single arrays, and returns a single array.