SLERP for position + quaternion

[AlexanderFabisch]

Fixes #273

• Add pytransform3d.transformations.pq_slerp, which performs linear interpolation of position and SLERP of quaternion.
• Add connections between docstrings through "See Also" sections.

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (7e7ccf4) 100.00% compared to head (6faa9ac) 100.00%.

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Additional details and impacted files

@@            Coverage Diff            @@
##           develop      #274   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           30        31    +1     
  Lines         2540      2552   +12     
  Branches       294       294           
=========================================
+ Hits          2540      2552   +12     

Files	Coverage Δ
pytransform3d/rotations/_slerp.py	100.00% <ø> (ø)
pytransform3d/trajectories.py	100.00% <ø> (ø)
pytransform3d/transformations/__init__.py	100.00% <100.00%> (ø)
pytransform3d/transformations/_conversions.py	100.00% <ø> (ø)
...m3d/transformations/_dual_quaternion_operations.py	100.00% <ø> (ø)
pytransform3d/transformations/_pq_operations.py	100.00% <100.00%> (ø)
...ansform3d/transformations/_transform_operations.py	100.00% <ø> (ø)
pytransform3d/uncertainty.py	100.00% <ø> (ø)

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