ds.asFactor when generating dummy variables, avoid generic DV prefix

[tombisho]

If you want to create dummy variables for multiple factor variables which both have levels 1,2,3 then both of these have the dummy variables DV2 and DV3. This makes it harder to recombine these back into a data frame as there is duplication of column names. It is not trivial to change column names

I recommend using the variable name as the default prefix and allowing the user to specify their own if they want.

For example for EDU_LEVEL and INCOME_LEVEL the dummy variables would be EDU_LEVEL_2, EDU_LEVEL_3 and INCOME_LEVEL_2 and INCOME_LEVEL_3