Add support for coordinate based slicing

[markspec]

Problem

Datasets may contain both Coordinates and Dimension Coordinates. Currently we only support slicing datasets along the Dimension Coordinates.

Solution

Add support for slicing based on a single coordinate. E.g. read a single inline from a post-stack volume. Support for both coordinates and dimension coordinates will simplify use in downstream applications.

Tests

• coordinate (e.g. inline) is not a dimension (e.g. worker_id/task_id/trace/sample)
• coordinate (e.g. inline) is a dimension (e.g. inline/xline/depth)

[BrianMichell]

Implementation

Viewing a "data" variable in the relation of its coordinates requires transforming and sorting of data. For that reason we will create a special CoordinateView class that will contain only the selected coordinate values. This will be similar to the sel method for Datasets but the view will be only in-memory.

Data structures

CoordinateView

• In-memory representation of a "data" variable oriented to coordinates.

DatasetView

• A helper class that will construct the CoordinateView.
• User specified read and cache policy.
  • Never cache
  • Lazy read + lazy cache
  • Eager read + lazy cache
  • Eager read + eager cache
• Cache
  • Different levels of sort order cached (flattened, chunk order, no sort)
  • Coordinate values may be cached lazily or eagerly.
• Provides user-extensible searching for custom orchestration.
• Does not support dimension-coordinate slicing (yet).