epi.{c,h} and the test suite 

Please, review and comment on the pull request that brings into `liboptv` the epi.{c,h}. These functions would have to be updated later when img_coord.{c,h} will be accepted. At the moment I made a shortcut in order to continue, Partik that works on img_coord can actually use the way I suggest to implement the function with the two new arguments.