warn the user when using sub-optimal PBFs

[cldellow]

Tilemaker's documentation suggests that users can get PBFs from BBBike:

tilemaker/docs/VECTOR_TILES.md

Line 25 in 1da4be9

You'll mostly use OpenStreetMap's openly-licensed map data to make your vector tiles. You get OSM data in a .pbf-format dump, rather than pulling it from an API. Several sites, such as [Geofabrik](https://download.geofabrik.de) and [BBBike](https://extract.bbbike.org), provide free country/city extracts of OSM data. (You can also download a dump of the whole planet [directly from OSM](https://planet.osm.org), but this is a massive file which is probably too much for tilemaker to process.)

BBBike uses osmconvert to generate PBFs. Reading through the osmconvert code 1, osmconvert will pack up to 31 MB of data into each block.

PBFs from Geofabrik use Osmium, which defaults to packing 8,000 objects into each block, resulting in blocks that are more like ~60 KB in size.

More, smaller blocks are better for Tilemaker:

• fewer blocks means less opportunity to use multiple cores
• larger blocks means higher baseline memory requirement

I emailed BBBike's maintainer. He wants to one day move to Osmium, and is understandably not keen on patching osmconvert in the interim.

He did point out that a user can just run osmium cat on a PBF from BBBike to rejig its innards.

This PR detects when a PBF is suboptimal, warns the user, and provides an explanation of how to fix it.

For one of my BBBike PBFs, this results in processing time dropping from 90 seconds to 28 seconds. (osmium cat itself only takes 17 seconds, and in any case, only has to be run a single time.)

[systemed]

Good spot - I pretty much exclusively use Geofabrik PBFs so had never noticed this!