Add HW17.4, HW18, HW19 implementations + full report

[Ash-Sdh]

This PR adds the complete implementation for the final homework tasks:

1. HW 17.4 — Added Implicit Euler, Crank–Nicolson, RC Circuit Solver
2. HW 18 — Implemented forward-mode Automatic Differentiation (AutoDiff)
3. HW 19 — Added explicit RK2 timestepper (required by assignment)
4. Added all test programs:
5. • test_ode.cpp
6. • test_rc.cpp
7. • test_autodiff.cpp
8. • test_pendulum_autodiff.cpp
9. Added plot generator for RC circuit (plot_rc.py)
10. Added REPORT-2.md with all plots and explanations
11. Added all report_images/ (mass–spring plots, RC plots)
12. Updated CMakeLists.txt to include new executables

Notes:
• RK4 was used only during development and has been removed (not required).
• nanoblas submodule was not modified.
• All added files are self-contained and safe to merge.