began a refactor of SGD.
refactored fuction factories to return functions that rely on argument 'data'.
cleaned up helper-mixtureModel to move functions into separate
helper-* files.
updated DESCRIPTION to include suggestion of libraries used to
generate and analyze mixtureModel data.
wrote test scripts for mixtureModel example and others to test SGD versus optim.
updated mixtureModelLogLikelihood.rda file to have 200x200 resolution.