Feat: Upgrade Vanderbilt code to Python 3 and add unit tests (Fixes #102)

[Aarshpatel12]

Summary

This PR upgrades the Vanderbilt University Medical Center (DS_BW) contribution to Python 3 and adds a comprehensive test suite.

Changes

• Python 3 Upgrade: Updated dcemri.py syntax (fixed print statements, updated imports).
• Dependency Fix: Updated skimage imports and scipy.integrate functions (cumtrapz -> cumulative_trapezoid) to support modern SciPy/Skimage versions.
• New Tests: Added test/test_DS_BW_VanderBilt.py covering:
  • tofts_integral
  • ext_tofts_integral
  • signal_to_noise_ratio
• Dependencies: Added scikit-image to install_requires in setup.py (matching project standards) and removed the temporary requirements.txt.

Verification

• Ran pytest locally.
• Result: 3 tests passed.