API changes in 'ks' package breaks flowStats

[mikejiang]

The following two APIs were moved from ks to feature package lately, which causes the error when building flowCore vignette.

Error : objects ‘dfltCounts’, ‘drvkde’ are not exported by 'namespace:ks'

However feature package requires tcl/tk, which is the dependency we wanted avoid due to the discussion here.

These two functions are indirectly used by curv2Filter so we can''t simply drop them unless we want to deprecate curv2Filter.

I will see if I can make hardcopies of these two functions from features without much hassle.

[mikejiang]

It seems also has impact on openCyto::tailgate which directly uses drvkde for kernel density derivative estimation.