Convenient methods in RasterLayer

[wang-boyu]

What's the problem this feature will solve?

1. Load/save raster cell values directly from/to files.
2. Get random coordinates within cells based on cell coordinates. In situations like population in the same cell, they don't need to have exactly the same coordinates.

Describe the solution you'd like

1. Have a RasterLayer.from_file(file_path, name, compression) class method to create RasterLayer with cell attribute name. The file can optionally be a gzip file.
   Similarly for saving to files: RasterLayer.to_file(file_path, compression).
2. Implement a raster_layer.get_random_coord(cell.pos) method.

Additional context

See comments from #90 (comment) and #90 (comment).

[wang-boyu]

Point 1 is partly addressed in #92. The support for compressed file is to be implemented in a future release.

For that we can probably take a look at Pandas' implementation on compression='infer' in read_csv. Source code that might be related:

• https://github.com/pandas-dev/pandas/blob/e8093ba372f9adfe79439d90fe74b0b5b6dea9d6/pandas/io/common.py#L602
• https://github.com/pandas-dev/pandas/blob/e8093ba372f9adfe79439d90fe74b0b5b6dea9d6/pandas/io/common.py#L252
• https://github.com/pandas-dev/pandas/blob/e8093ba372f9adfe79439d90fe74b0b5b6dea9d6/pandas/io/common.py#L453
• https://github.com/pandas-dev/pandas/blob/e8093ba372f9adfe79439d90fe74b0b5b6dea9d6/pandas/io/common.py#L462
• https://github.com/pandas-dev/pandas/blob/e8093ba372f9adfe79439d90fe74b0b5b6dea9d6/pandas/io/common.py#L498

[rht]

Also, to_file or something would be useful.