[ros_mary_tts] German voices

* Installed two male German voices. Closing issue: https://github.com/strands-project/memory_game/issues/47 and #20
 * The standard voice is still female and english (Linda's voice) but it is easily reconfigurable via service calls (See README)
 * The memory_game does the reconfiguring itself so for the Vienna set-up nothing has to be changed in the standard launch files.
* Removed some of the downloaded files that are just taking up space.
* Adapted the voice and language switching to voices that do not use specific configuration files like the previously installed ones and therefore made it a bit more generic.
 * This allows for easy install of additional voices (See README)