Consider returning a `SummarizedExperiment` object from `digestFastqs`

[csoneson]

Rather than a list, we could consider returning a SummarizedExperiment object directly from digestFastqs. These could then be combined across samples with the combineCols() function. We would need to provide a sample name to digestFastqs, to use as the column name in the assays and to name the parameters in the metadata (since these will be combined without name checking by combineCols()). We can have multiple assays (reads, maxNReads, UMIs if applicable). This would also mean that we could use the existing QC plot functions directly on each sample as well.

[csoneson]

From a first look, it seems that combineCols() (or more precisely, combineUniqueCols() applied to the rowData of the respective SEs) will not behave exactly like what we are currently doing. For example, it seems that combining list columns is not currently supported, and even if it was, in cases where the same rowData column name appears in both objects but the values are not the same, the column from the first object will be used, and that from the second one will be discarded.