Add CurveFitter with scipy least_squares, configurable loss functions
(soft_l1, huber, cauchy), profile likelihood, confidence intervals, and
goodness-of-fit metrics. Add CWExtractor for extracting shape factors
from beta spectra via Kurie analysis, parametrization fitting, and
g_V/g_A extraction. Includes 32 new tests (18 fitter + 14 CW extractor).
Bump version to 0.3.0.
