allow syberia stages to be built using arity 1 functions

@tonglu Evaluation stage builder can now accept just `modelenv` or just `evaluation_options`. For example,

````R
evaluation_stage <- function(modelenv) {
    ... # will have access to modelenv, but ignore any passed options
}

evaluation_stage <- function(evaluation_options) {
   ... # will have access to evaluation_ops, but ignore the modelenv.
       # This is usually the right thing to do!
   function(modelenv) ...
}