Fix bug in admittance controller access to reference interfaces

[marip8]

This PR fixes an issue in the admittance controller where it always tries to access both the position and velocity reference interfaces even if one of them is not provided. Currently, if the chainable_command_interfaces parameter only includes position or velocity, the admittance controller will complain with the following message:

Export of the state or reference interfaces failed with following error: The internal storage for reference values 'reference_interfaces_' variable has size '6', but it is expected to have the size '12' equal to the number of exported reference interfaces. Please correct and recompile the controller with name 'admittance_controller' and try again.

To fix this issue, the controller should only access the reference interface(s) that match its chainable command interface(s)