Add get_recall to TrecEval; not test yet

[hyusterr]

Hi,
As title, since it is too sad to see an IR tool Iacks recall(jkjk), I implemented a raw version of get_recall into TrecEval by mimicking the implementation of get_precision and get_rprec. However, I did not find a guideline or sth similar to test my implementation. Could you provide me with some guidelines to test my implementation?