Split downloading the MEDS dataset to a separate file

downloading all the datasets in downloadDatasets.sh is time consuming, and
MEDS is the only dataset used in make test, as well as
evalFaceRecognition-MEDS.sh. Splitting MEDS to a searate file simplifies
the process of validating an OpenBR build.
Merge pull request #183 from biometrics/split_download

Split downloading the MEDS dataset to a separate file