Extend RoI metric for categorical values

In order to measure the richness of information for fields like 'tags'
fields I have extended the metric. Now categorical values are counted
based on the field path and their value. This commit adds a test to
verify that the counting works as expected.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Complete richness of information for categories

The richness of information for categorical values is expressed as
negative probability of its occurrence assuming normal distribution.
I have added a unit test for this behavior as well as completed the
implementation to serve this.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>