argparser/etc.: Implement 'must' for value restrictions
argparser: Add isParsed, parsedValues, correct comment
All apps: Eliminate handle_parameters(), check_usage(); incorporate into Params classes
Perceptron: Implement --perceptron-decay
Rerank/Cleanup: Rename 'candidate instance' -> 'candidate feature vector', etc.
Rerank: --rerank-classifier -> --rerank-optimizer, add stub for MLogit
print: Add errfmt() for formatting as errprint() does
Classifier/memoizer: Add code to read non-nominal data more flexibly, including data with choice-specific features; implement appropriate classifiers; change defn of memoizer:number_of_valid_indices, add maximum_index