Extend `countData` to handle nested files with counts and features from a single file, `data.frame` or `data.table`

[agusinac]

Often you receive the featureData and countData combined into a single file, now the user needs to manually split it making this extra layer of data wrangling less user-friendly. Since the metaData is always given, we can assume that all samples matching SAMPLE_ID belong to the countData and everything else can be placed in the featureData.

This same function can also be used to not only import but also write files in similar format. Thus the omics class can use a write function that outputs the metaData and combined countData & featureData as separate files.

• Create private helper function to extract and fuse countData and featureData
• Add public function write to output files ( optionally add compression? )