Safe predictions using `@formula` and regression splines

[mclements]

[Apologies for cross-posting]

I put the following issue on Discourse, which may better belong here: Safe predictions using @formula and regression splines.

In outline, the knots from Splines2.ns() need to be captured from the original data to allow for "safe" predictions. This may be related to Issue #154. Any suggestions?

Sincerely, Mark.