Implement --index-parts for shapefiles

[GretaCB]

Add Mapnik's --index-parts flag for all shapefile preprocessing.

To quote @BergWerkGIS 's comments:

It's useful:

• if there are any multipart features at all
• the multipart features have a big bounding box, but also contain small features

--index-parts should help if there are multipart features like this:

 ______________________________
|   _       _   _              |
|  |_|   _ |_| |_|             |
|       |_|                    |
|                              |
|             _                |
|      _     |_|               |
|     |_|                      |
|                              |
|                _   _         |
|               |_| |_|        |
|     _             _          |
|    |_|           |_|         |
|                              |
|______________________________|

And to quote @springmeyer 's comments:

My understanding is that --index-parts is fine to use in all cases. No major slowdowns on non-multipart features are anticipated, but speedups for multipart features should be seen.

[mapsam]

@GretaCB looks like this can be closed since https://github.com/mapbox/preprocessorcerer/pull/59/files is merged?

[GretaCB]

👍 totes, thank you! Closing.