Add transforms to new_convolve_scanner, make identity transforms default for new_double_scanner

[dylanhmorris]

This will:

1. Align the APIs
2. Make it possible to implement logistic susceptibility transforms using new_convolve_scanner Actually the specified change won't suffice for this b/c the logistic susceptibility adjustment needs cumulative incidence, so it will need a different (heavier) scanner.

Out of scope

• an n-fold convolve scanner for arbitrary n is worth exploring but out of scope for now.
• a scanner that keeps track of cumulative incidence