Next IMEX issue

[maggul]

solver:imex=false flag was causing a potential error when solver:type=arkode was used, this is now fixed. Also, now omits creation of implicit algebraic solver when running with an explicit solver.

When solver:imex=false and solver:implicit=true and solver:explicit=true, then the module now sets solver:imex=true and proceeds with this option. If you prefer returning an error instead, just let me know.