Jtc sum periods

[fmauch]

Sample trajectory based on the sum of periods instead of the absolute time.

This is basically the second part of #1191. In the long run I want to be able to scale the period with a scaling factor in order to artificially slow down / speed up progression in the trajectory. This PR is there to separate the necessary time counting change from the scaling functionality to make things more contained and easier to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.60%. Comparing base (9b344c7) to head (17adfc3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1395      +/-   ##
==========================================
+ Coverage   83.58%   83.60%   +0.02%     
==========================================
  Files         122      122              
  Lines       10984    10988       +4     
  Branches      936      934       -2     
==========================================
+ Hits         9181     9187       +6     
  Misses       1490     1490              
+ Partials      313      311       -2     

Flag	Coverage Δ
unittests	83.60% <100.00%> (+0.02%)	⬆️

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

Files with missing lines	Coverage Δ
...jectory_controller/joint_trajectory_controller.hpp	100.00% <ø> (ø)
...ory_controller/src/joint_trajectory_controller.cpp	83.90% <100.00%> (+0.04%)	⬆️
...ectory_controller/test/test_trajectory_actions.cpp	97.77% <100.00%> (+0.02%)	⬆️

... and 3 files with indirect coverage changes