Add getContainerCount() method

[lemire]

Is your feature request related to a problem? Please describe.
In some instances, we want to sort bitmaps with respect to the number of containers they hold. See #608 (comment)

Describe the solution you'd like
We should add a function to our public API that gives convenient access to the number of containers.

It would be as trivial as return highLowContainer.size(); though we want to cover both RoaringBitmap and ImmutableRoaringBitmap (and maybe the 64-bit bitmaps as well?). Proper documentation should be included.

[sladyn98]

@lemire Any good places to start ? I would like to contribute to this issue

[lemire]

@sladyn98 Good! Look at how we compute the cardinality. Computing the number of containers is similar/simpler.