Add drop and type to generic signature of [row|col]Quantiles

[const-ae]

After some discussion with @LTLA in const-ae/sparseMatrixStats#7, I decided that it is probably a good idea to include drop and type (which sparseMatrixStats now fully supports) in the generic method signature.

What do you think?

[hpages]

Well, as a general rule, if matrixStats::rowQuantiles has the arguments, the generic should probably have them too.

[hpages]

I'm talking about arguments that make sense for matrix-like objects in general. Not all arguments fall in that category but it seems that drop and type do.

[PeteHaitch]

Where do we sit on this?

[const-ae]

I rebased the PR on the current master to resolve the merge conflict. I think we can merge this now :)

[PeteHaitch]

Thanks, @const-ae!