Speed up streambugs.get.sys.def

In the run.streambugs calls (with C ODEs simulation) ca. 70% and ca. 57% of the time, respectively, for minimum and extended case study is spent in the pure R function streambugs.get.sys.def (most in the big for loop therein).

streambugs.get.sys.def function depends on passed in parameter values (takes y.name,par,inp from run.streambugs inputs). Optimise speed for use case of sampling parameter values.