Add support for missing-ness

[Dayne Filer, MD, PhD]

Add functionality to track missing-ness in TxpResult object.

• Add txpMissing slot to TxpResult class; observation by slice matrix (same as txpSliceScores ) with the proportion of missing observations
• Update txpCalculateScores to calculate missing-ness
• Add accessors for txpMissing
• Update man page and create examples showing accessors
• Add tests to cover new missing-ness functionality

[David Reif]

Completed with latest update from /ggplot branch