Allowing stddiffs for continuous variables to be negative

This pull request consists of code realignment for readability and a minor modification to allow the standardize differences for continuous variables to be negative.

The standardized differences can be negative in the original stddiff SAS macro, as seen in the online documentation (http://support.sas.com/resources/papers/proceedings12/335-2012.pdf; Page 5; Table 3). It is implemented as follows.

```
(a._mean_ - b._mean_) / sqrt((a._std_**2 + b._std_**2)/2)
```