Add `tile_multiplier`

[zstadler]

Expand a list of changed tiles to higher and/or lower zoom levels.

This program expands the list of tiles produced by import-diff to be used by generate-tiles. import-diff produces a list of tiles in zoom MAX_ZOOM that need updating. When generate-tiles is used with FILE_LIST, it expects a list of tiles in all zoom levels.

• For each line of the form z/x/y in its standard input, representing a tile, the program outputs all tiles that overlap it for all zoom levels between <min_zoom> and <max_zoom>.
• Each tile will appear once in its output.
• The input zoom level need not be within the <min_zoom>..<max_zoom> range.

Thanks @frodrigo for https://github.com/makina-maps/makina-maps/blob/master/nginx/tile_multiplier.py

[zstadler]

Related to #290, #313, #401

[TomPohys]

Thank you!