TensorFlow port

[cjekel]

Needed:

• [x] assemble_regression_matrix
• [x] fit_with_breaks
• [x] fit_with_breaks_force_points
• [x] predict
• [x] fit_with_breaks_opt
• [x] fit_force_points_opt
• [x] standard_errors
• [x] prediction_variance
• [x] r_squared
• [x] fit
• [x] fitfast
• [x] fit_guess
• [x] use_custom_opt

Probably won't change:

• [x] calc_slopes
• [x] p_values

Add errors/warnings: - [ ] small number of data - [ ] square matrix I added try except catches to the optimization functions that would result in either a failed Cholesky decomposition or non-invertible matrix.

[cjekel]

https://github.com/cjekel/piecewise_linear_fit_py/pull/25