fixed admittance_controller/AdmittanceController::read_state_from_hardware()

[tpoignonec]

Issue: only one state interface is read, but at least position AND velocity should be read.

Cause: "else if" statements used instead of "if" in AdmittanceController::read_state_from_hardware().

Solution: see edits.

[tpoignonec]

N.B., actually this seems to be common to all branches (e.g., see main branch).

[bmagyar]

I tried simply retargeting the PR but it isn't that simple unfortunately. Please rebase this PR to the master branch and we can backport it to humble.

[bmagyar]

Additionally, if you could add a test that reproduces the issue that'd be great

[tpoignonec]

OK thanks, will do.

[tpoignonec]

Actually, we can probably just close this PR, I hadn't realized the controller was made for position interface specifically.
After investigation, my issue was more due to the fact that I tried to modify the code to do something it wasn't designed to do...

@bmagyar
If I find a test scenario, I'll reopen + rebase on main as you requested.
Sorry for the inconvenience.