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 85.26%. Comparing base (7b7ecb7) to head (d3e7bb6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1913   +/-   ##
=======================================
  Coverage   85.26%   85.26%           
=======================================
  Files         143      143           
  Lines       13793    13793           
  Branches     1196     1196           
=======================================
  Hits        11760    11760           
  Misses       1636     1636           
  Partials      397      397           

Flag	Coverage Δ
unittests	85.26% <ø> (ø)

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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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