fista.cpp and fista_cox.cpp now have internal linkage (anonymous
namespaces).coef() now returns a one-column sparse matrix instead of a two-column data frame.
Zero coefficients are printed as ., which keeps the display compact in high dimensions.
Added a summary() method for pic fits.
intercept = FALSE is now permitted for every family. Previously it raised
an error for non-Cox families when standardize = TRUE. When
intercept = FALSE, the data are assumed to be centered.
Standardized the input-validation error messages (consistent back-quoting
of argument names and call. = FALSE).