Fast updates of parameter values assuming identical parameters names

Parsing of parameters specification via streambugs.get.sys.def is still too slow (cf. this note) and other than getting parameter values unnecessary for applications likes parameter values optimisation or sampling.

Instead, for a fast update of parameter values a model state containing mapping of indices of parameter vectors into elements of constructed system definition could be saved.

This most likely would practically make further performance optimizations of streambugs.get.sys.def like #12 unnecessary.