Click Model Implementation

Hi,

Now that the clickstream logging PR (#161) is close to getting merged
I'm starting to move onto the next step i.e. the implementation of DBN
click model for which I think it'd be best to discuss the approach that
I intend to take and the classes required.

I'm thinking of starting with the implementation of a base class Model
to enable the implementation of DBN click model as its subclass
and more click models classes in the future.

DBN model can be implemented as two classes, simplified DBN model
class (SDBM) amd the normal DBN class (DBN). Also, as discussed
during the meeting earlier, I would be focusing on the SDBN class first.

And I was wondering if it'd be recommended to open a small PR
with the implementation of the base class as soon as it's ready or
just a single PR later complete with the implementation of SDBN
model?

Thanks,
Vivek