Add precision parameter to all toGeoJSON functions

[mattgrande]

As recommended in PR #5237. Example usage:

var marker = new L.Marker([10.123456, 20.123456, 30.123456]);
marker.toGeoJSON();    // Works without changes
marker.toGeoJSON(3);   // Restricts all numbers to three significant digits

[perliedman]

Hi and thanks a lot for looking into this!

I have three points I'd like to see addressed before merging this:

• As it is now, the coordinates output by toGeoJSON will be strings, not numbers; for example ["-87.359296", "35.00118"] rather than the expected [-87.359296, 35.00118] - this looks wrong and might possibly trip up some receivers that care about data types; it is also against the RFC 7947 which says: "A position is an array of numbers"
• The GeoJSON spec gives the precision as number of decimals, while this PR gives the precision as number of significant digits; as it is now, it is not possible to give a number that adheres to the GeoJSON spec's recommendation of six decimals
• Not necessarily important, but I think it might also be a good idea to default to the GeoJSON specs recommendation, so that we use six decimals unless something else is specified.

[mattgrande]

Hmm, looks like toFixed would work better than toPrecision... but that also returns a string, and if I convert to a number, I'm getting weird results. Will continue to look into it.

[fminuti]

I think you could use L.Util.formatNum

[mattgrande]

Sorry for the delay, @perliedman! Those changes you requested are in.

[fminuti]

this way you cannot round to 0 decimals. When (if) my pr #5492 lands, you can skip this line of code.

[mattgrande]

Oh, good catch, I'll update that. I'm going to leave the default of 6 for now, though, because I don't know when/if your PR will come through.

[perliedman]

Looks good! Thanks for finishing this!