Allow setting "init(k1)" when k1 has an initial assignment.

[luciansmith]

An initial assignment is used to initialize values in an SBML model. Similarly, "r.setValue('init(k1)')" is the method in roadrunner that sets the 'reset' value of a parameter.

Since these are essentially the same things, I think that r.setValue('init(k1)') should reset the initial assignment of the variable in question (right now, roadrunner just refuses to do it at all).

@matthiaskoenig: would this affect you guys at all? Herbert wanted to check before implementing this.