CostTerms: joint-weighted costs, expose to python

[rhaschke]

This implements joint-specific weighting of PathLength and DistanceToReference (new) costs.
Also expose cost stuff to python.

[rhaschke]

Hm. The segfaulting tests seem to stem from pybind11 issues in a Debug build: pybind/pybind11#4395

[codecov]

Codecov Report

Base: 59.03% // Head: 58.97% // Decreases project coverage by -0.07%⚠️

Coverage data is based on head (8faf416) compared to base (de22cba).
Patch coverage: 39.03% of modified lines in pull request are covered.

❗ Current head 8faf416 differs from pull request most recent head 7c35f6c. Consider uploading reports for the commit 7c35f6c to get more accurate results

Additional details and impacted files

@@ Coverage Diff @@## master #409 +/- ##
==========================================
- Coverage 59.03% 58.97% -0.06% 
==========================================
Files 94 94 Lines 8322 8436 +114 ==========================================
+ Hits 4912 4974 +62 - Misses 3410 3462 +52 

Impacted Files	Coverage Δ
core/include/moveit/task_constructor/task.h	25.00% <ø> (ø)
core/src/cost_terms.cpp	24.12% <4.17%> (-8.69%)	⬇️
core/python/bindings/src/core.cpp	87.88% <87.50%> (-0.32%)	⬇️
core/include/moveit/task_constructor/cost_terms.h	85.72% <100.00%> (-14.28%)	⬇️
core/python/bindings/src/core.h	54.17% <100.00%> (-9.46%)	⬇️
core/python/bindings/src/solvers.cpp	93.11% <0.00%> (-2.89%)	⬇️
core/include/moveit/task_constructor/cost_queue.h	97.68% <0.00%> (-2.32%)	⬇️
core/include/moveit/task_constructor/storage.h	86.96% <0.00%> (-0.95%)	⬇️
... and 9 more

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