Add a summary option to summarizeMolecularProfiles acting on features

[p-smirnov]

For microarray data (or any data really), we would like to be able to summarize rows of molecular data by a user provided function (mean/median or a custom function), based on grouping by any column in the featureData for that molecular profile.

The use case is when we have multiple probe per gene, to be able to quickly summarize to a single value per gene when extracting the data out of a PSet.