Use must initialize the default_* variables in model. These variable must be
copied into real model's variables. The default_* variables are used to
initialize and re-initialize models.

K-H-Ismail authored Apr 02, 2020 and Gauthier Quesnel committed Apr 03, 2020

This model permits through a lambda function to applyt t -> f(t).

- adding tests for time_func model and adpting Izhikevitch simulation
- update public-api.cpp