Disables roaring imports when trackExistence is true

[yuce]

Fixes #238

• Adds Field.HasRoaringImport method
• Disables roaring imports when trackExistence is true

[jaffee]

Instead of making this check on the client side, I wonder if we should...
(a) disable this server side, or
(b) allow it, and also expose the ability to roaring import into the existence field. Basically, if you're using roaring imports, and existence tracking, you're responsible for updating the existence field, which in most cases can probably be done with very little overhead by importing full RLE containers in your known column range.

I'm leaning toward (b), though this probably requires a bit of work in Pilosa.

[yuce]

Both approaches are possible, but this PR is more like a very quick way to avoid confusion for go-pilosa users.