Compute per cell metrics (and per feature?)

[cvanderaa]

Let's have a function that computes a metric or statistic for every cell across several batches. The function would be a generalization of computeMedianCVperCell().

Tentative signature: computePerCell(object, i, colDataName, FUN, ... ), where colDataName is the names of the column in the colData to store the metric, and FUN is the function that computes the metric for one cell/sample/column, with its arguments passed through ....

I'm not convinced by the function name... The scuttle package implements the perCellQCMetrics method for SE and SCE objects, but the signature differs from what I want and does not allow for custom functions.

Also, we could have a similar function that computes metrics per feature. This would be a generalization of computeSCR.