This version:
- Adds null/total counts for every variable
- Replaces variable code with label
- Removes unnecessary lower/upper confidence intervals

…38/descr-stats-show-nulls

Algorithms now accept a new formula argument. This arg expects a
description of a formula in Wilkinson notation in JSON format. From the
description an actual string formula is constructed and passed to the
patsy library from which the algorithm's design matrices are
constructed. The rest of the execution remains unchanged.

Currently only a subset of the full Wilkinson notation is implemented.
Interactions are up to 3 terms and singleton terms have a limited number of
transformations allowed. These are nop, log, exp, center, standardize,
mul and div for numerical variables.

Currently logistic_regression tests with formula are failing. There is a
subtle interaction between the formula coding of categorical
variables and the the add_missing_levels method. The reason is that
add_missing_levels is based on the known enumerations of categorical
vars found the in the CDEs but the codings supported by the formula
might add different enums to categorical vars.

Working version of descriptive_stats with an optional formula parameter.
The formula applies only to model results as it make little sense in
single variable results. Moreover, all floats in the result are now
rounded to two decimals for pesentation, and the tests have been
corected accordingly.

…nto dev/518/formula