Osyris

- Fix bug when trying to load empty sink files
- Fix `lmax` being ignored when filtering on levels is requested on `select`
- new sample data for docs.

- New data structure: top level `Dataset`, which contains `Datagroup`, which contains `Array`. Groups are `amr`, `hydro`, `grav` etc...
- Added scatter plots (can be added as layer to plane plots)

- Add `cpu_list` argument to load function to enable selective loading of cpu files: `data = osyris.load(71, scale="au", path="data", cpu_list=[5, 9, 20, 30, 60])`

- Add hilbert based loader, when selective loading is carried out on the x,y,z coordinates: `data = osyris.load(71, scale="au", select={"x": lambda x : x > 5500. * osyris.units('au')})`

- `lmax` has been removed in arguments of `load`, instead use `data = osyris.load(71, scale="au", select={"level": lambda l : l < 12})`

- New algorithm for plane plots: hybrid method with first histogramming and then filling the blanks by finding the actual cell in the 3d domain that contains the blank image pixel.

- fix bug in Array subtraction
- use new method for making plane plots based on histogramming and scipy distance tranform

New features:

- More Pythonic API
- Runtime physical units using pint
- Arithmetic operations between data arrays
- More generic interface for plots (easier to use custom matplotlib options)
- Cleaner data loader

Tag of version 1.0, very close to the original version of osyris.