fixed LiZhong's Markhov model SVD (PCA) experiment
add character class that includes all ascii characters
function to calculate the histogram of "invalid" characters at each posiiton in a string. fix double-import of regular_expressions
fix bulk_update
fix comments and doctest strings for index_model_field and _batches
change pipesale to sale in a Refrefurb view (why is that here?)