expose get_data_type method in loaned interfaces

[saikishor]

This is needed if the user wants to query in the controller code

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.96%. Comparing base (2e82c40) to head (ec160b0).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2351      +/-   ##
==========================================
- Coverage   88.99%   88.96%   -0.03%     
==========================================
  Files         148      148              
  Lines       16805    16805              
  Branches     1438     1438              
==========================================
- Hits        14955    14951       -4     
- Misses       1287     1289       +2     
- Partials      563      565       +2     

Flag	Coverage Δ
unittests	88.96% <100.00%> (-0.03%)	⬇️

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

Files with missing lines	Coverage Δ
.../include/hardware_interface/actuator_interface.hpp	91.30% <100.00%> (ø)
...de/hardware_interface/loaned_command_interface.hpp	52.50% <ø> (ø)
...lude/hardware_interface/loaned_state_interface.hpp	60.00% <ø> (ø)
...ce/include/hardware_interface/system_interface.hpp	84.80% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[christophfroehlich]

LGTM. should we add a test for the set_command methods with other types than double?

[saikishor]

LGTM. should we add a test for the set_command methods with other types than double?

The API behind it is already tested, so mostly not needed.