Reset PID controllers on activation and add save_iterm parameter

[christophfroehlich]

Since ros-controls/control_toolbox#180 there is a parameter to control the behavior of retention of the integral term after reset.

Actually, we only have reset in on_configure here. I added the reset to on_activate() and documented the parameter in the GPL yaml.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.20%. Comparing base (af572aa) to head (fff4e8a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1507   +/-   ##
=======================================
  Coverage   84.19%   84.20%           
=======================================
  Files         123      123           
  Lines       11321    11324    +3     
  Branches      955      957    +2     
=======================================
+ Hits         9532     9535    +3     
+ Misses       1474     1473    -1     
- Partials      315      316    +1     

Flag	Coverage Δ
unittests	84.20% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
pid_controller/src/pid_controller.cpp	68.69% <100.00%> (+0.27%)	⬆️

... and 3 files with indirect coverage changes