Add test for document frequency and refactor

I have added a test for the document frequency computation. The test
revealed a flaw with the initialization of the hash which is fixed by
this commit.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add last test and refactor richness of information

I have added the last test which completes the richness of information
specification. For that I have also refactored the richness of
information metric code.

I changed the computation a bit. What still lacks is the summation of
different field scores of a document.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Remove document length tracking from RoI metric

Since the richness of metric code became obsolete I have removed the
tracking of the document length and code shorten the code a bit.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Remove document length tracking from RoI metric

Since the richness of metric code became obsolete I have removed the
tracking of the document length and code shorten the code a bit.

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