proper support for repeated events everywhere in the library

[zenogantner]

In some domains, we have repeated events, in some domains we have not.
Most recommenders and evaluation protocols (except for item recommendation) in MyMediaLite implicitly assume that e.g. a rating for a given user-item combination can only occur once. This is of course not always valid, in particular if we have time stamps etc.

In every place in the library and its tools it should be clear and documented whether repeated events are possible or not.
If there is not reason to not support them, they should be supported, and the user should be able to enable them.

We should also have methods that are particularly suited to repeated events, see e.g. #29
#124 is related

[zenogantner]

This also includes support (data structures, I/O, recommenders) for feedback counts.

Example dataset: http://labrosa.ee.columbia.edu/millionsong/tasteprofile

=> started a new ticket for this: #266