Improve Linear Regression: Use ndarray-linalg's lstsq, improve names and follow conventions

[paulkoerbitz]

There are some improvements to the linear regression / least squares code which would be nice to implement.

• Use least squares from ndarray-linalg (lapack), now that it is exposed
• Address comments from original PR
  • Fix names fit_intercept, fit_intercept_with_normalisation to be consistent with linfa-logistic
• Follow C-GETTER for getter names