[WIP] Add ComponentPath overrides to Component interface

[adamkewley]

… and integrate into Millard muscles and scalar actuators

Brief summary of changes

• Adds ComponentPath overrides for path-dependent parts of Component's public interface
• Changes Millard muscles, and scalar actuators to use ComponentPath, rather than std::string, so that the path is pre-parsed ahead of time, which can improve performance
• Performance numbers WIP

Testing I've completed

• Existing test suite
• Basic tests for overloads WIP

Looking for feedback on...

CHANGELOG.md (choose one)

• WIP

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This change is 

[adamkewley]

Speedup appears to be roughly 5 % on Rajagopal, which uses MillardEquilibriumMuscle:

Test Name	lhs [secs]	σ [secs]	rhs [secs]	σ [secs]	Speedup
ToyDropLanding_nomuscles	0.04	0.00	0.04	0.00	1.11
RajagopalModel	3.82	0.00	3.59	0.00	1.06
Gait2354	0.20	0.00	0.19	0.00	1.03
passive_dynamic_noanalysis	1.22	0.00	1.23	0.00	1.00
passive_dynamic	1.83	0.00	1.82	0.00	1.00
Arm26	0.17	0.00	0.16	0.00	1.03
ToyDropLanding	0.04	0.00	0.04	0.00	1.02