Align default type behaviour in plotting functions

[OllyButters]

The plotting functions are inconsistent in the way they handle type. Some are combine by default, others are split. Also, boxplot has pooled as it type. We should tidy these up.

https://github.com/datashield/dsBaseClient/blob/master/R/ds.boxPlot.R
https://github.com/datashield/dsBaseClient/blob/master/R/ds.histogram.R
https://github.com/datashield/dsBaseClient/blob/master/R/ds.scatterPlot.R
https://github.com/datashield/dsBaseClient/blob/master/R/ds.heatmapPlot.R

[StuartWheater]

This feels like a 6.4.0 or even a 7.0.0 release feature, not for 6.3.2 release.

Stuart

[OllyButters]

Agreed!