Feature/ros2control

[firesurfer]

This PR adds some necessary additions needed in order to use this package with ros2control.

1. -fPIC is needed as compile flag
2. Adds EthercatMasterSingleton -> This is the only way I can think of at the moment how one could use the same ethercat master instance in multiple hardware interfaces

TODO:

The EthercatMasterSingleton should also wrap the update of the handled EthercatMasters. otherwise multiple hardware interface will call update themselves in the read/write methods which leads to update being called too often. (Don't know if that would be an issue actually ?)

EDIT: I solved this by having the singleton actually spin each master in a seperate thread. This should work as all the ethercat sdks are threadsafe as far as I know (or at least they should be)

@tfabi and @janpreisig