Add derivatives for RegularizationSmooth and Curvefit

[sathvikbhagavan]

This PR does the following:

1. Add derivatives for RegularizationSmooth using derivatives of CubicSplines as it fits a CubicSpline after solving the optimization problem
2. Add derivatives for Curvefit using ForwardDiff
3. Added tests for both
4. Added extrapolate keyword to both to make it consistent with other methods
5. Reorganized the extension files for more consistency
6. Formatted the repo

Partial fix towards #175