Improved coordinate system

[hanjinliu]

Terminologies

• Axis is a named object with an Index. Implements "slice", "size" because Index implements these.
• Index defines how to measure an axis (scale, offset, labels etc.). Implements "slice" and "size".
• Space is a collection of multiple names. It is unsized. When a shape (and additional arguments such as scale and offset) is passed, it builds a Coordinates. Implements "ndim", "get_coords" and "get_axis". A Space may have a Metric that defines relationship between axes.
• Coordinates is a collection of multiple Axis objects. This is actually used in arrays.

Expected API

• In arr = cr.array([...], coords=coords), coords should be an object equivalent to Space, rather than a Coordinates.
• arr.coords should return a Coordinates because it is already sized.
• Setter arr.coords = ... can also be a Space.

[hanjinliu]

Space should also implement array constructors.

space = Space(["z", "y", "x"])
arr = space.zeros((10, 20, 30), dtype=np.float32)  # 3D array with zyx-axes

Specifying the type of axis may be useful.

space = Space(angle="periodic", channel="category", y="linear", x="linear")
arr = space.zeros((10, 3, 20, 30))
arr = space.zeros((10, 3, 20, 30), channel=["R", "G", "B"])  # ??