Make biotac driver fully backwards compatible

[toliver]

Recent changes in the biotac.cpp have made it incompatible with palm FW versions older than 2151.
This only affects hands with biotac sensors.
The symptoms are that only one every 2 electrode values are correct.

The proposed fix for this would involve checking the palm FW version inside biotac.cpp. Right now this information is not available at that level, so some extra information would have to be passed in. The version info is available in the slave handler. e.g in https://github.com/shadow-robot/sr-ros-interface-ethercat/blob/indigo-devel/sr_edc_ethercat_drivers/src/sr06.cpp#L191

sh_->get_revision()