Deprecate forward_command_controller specializations

[christophfroehlich]

In a recent PMC meeting we decided to deprecate the specializations for the sake of reducing duplicate code here in the repository.

Should we rename the package (can be done by adding a shim package with the old name and deprecation warnings) before adding another controller (twist_controller from #300)?

Slightly related to #157 (no need for controller specifications if we use standard Float64MultiArray message anyways)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.13%. Comparing base (f59b5a0) to head (4f9269c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@ ## master #1913 +/- ## ======================================= Coverage 85.13% 85.13% ======================================= Files 144 144 Lines 13968 13968 Branches 1201 1201 ======================================= Hits 11891 11891 Misses 1670 1670 Partials 407 407 

Flag	Coverage Δ
unittests	85.13% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
..._controllers/src/joint_group_effort_controller.cpp	62.50% <ø> (ø)
...ontrollers/src/joint_group_position_controller.cpp	66.66% <ø> (ø)
...ontrollers/src/joint_group_velocity_controller.cpp	62.50% <ø> (ø)

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[saikishor]

LGTM