Fix warnings in DC Motor PI tutorial

[ChrisRackauckas-Claude]

Summary

• Fixed initialization warnings by providing complete initial conditions for all state variables
• Added documentation note about Pluto-specific variable naming requirements
• Updated sensitivity analysis code to use unique variable names

Changes

1. Complete initial conditions: Added initial values for all state variables (L1.i, inertia.w, inertia.phi, pi_controller.int.x) to avoid underdetermined system warning
2. Pluto compatibility note: Added a note explaining that Pluto requires unique variable names and how to resolve "Multiple definitions" errors
3. Unique variable names: Renamed simplified_sys variables in sensitivity analysis to avoid conflicts (simplified_sys_S, simplified_sys_T, simplified_sys_L)

Test plan

• Created test script to verify changes
• Confirmed complete initial conditions eliminate underdetermined warning
• Verified sensitivity analysis runs without variable name conflicts
• Tested that incomplete initial conditions still produce expected warning

Fixes #410

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