Add non-trajectory executors to Robot Class

[egordon]

Previously, only a single TrajectoryExecutor was accessible from the Robot class.

This PR adds the functionality to have the Robot class manage multiple executors of arbitrary type (with convenience methods for the basic Trajectory, Position, Velocity, and Effort types).

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Before creating a pull request

• [x] Document new methods and classes
• [x] Format code with make format

Before merging a pull request

• [x] Set version target by selecting a milestone on the right side
• [x] Summarize this change in CHANGELOG.md
• [x] Add unit test(s) for this change

[codecov[bot]]

Codecov Report

Merging #614 (401d6b1) into master (bda3396) will decrease coverage by 0.16%. The diff coverage is 88.88%.

:exclamation: Current head 401d6b1 differs from pull request most recent head 4f10d4c. Consider uploading reports for the commit 4f10d4c to get more accurate results

@@            Coverage Diff             @@
##           master     #614      +/-   ##
==========================================
- Coverage   73.19%   73.03%   -0.17%     
==========================================
  Files         216      214       -2     
  Lines        7768     7661     -107     
==========================================
- Hits         5686     5595      -91     
+ Misses       2082     2066      -16     

Impacted Files	Coverage Δ
include/aikido/control/Executor.hpp	60.00% <0.00%> (ø)
include/aikido/control/util.hpp	100.00% <ø> (ø)
include/aikido/robot/Robot.hpp	100.00% <ø> (ø)
src/robot/Robot.cpp	22.43% <87.71%> (ø)
include/aikido/robot/detail/Robot-impl.hpp	100.00% <100.00%> (ø)
include/aikido/robot/util.hpp	66.66% <100.00%> (ø)
src/control/Executor.cpp	94.73% <100.00%> (ø)
...l/KinematicSimulationJointCommandExecutor-impl.hpp	93.42% <0.00%> (-1.65%)	:arrow_down:
... and 9 more