Save function pointers in model object

[kylebaron]

I think rather than making it work like this by default, instead let the user cache the pointers for when the model is getting run over and over.

1. Helper functions to setpointers() and getpointers()
2. Put C++ code in mrgsolve.cpp
3. Put function in mrgsolve_init.cpp
4. Optional refresh
5. Modify loadso()
6. Modify mread()
7. Modify mread_cache()

// [[Rcpp::export]]
bool VALIDPOINTERS(Rcpp::List x) {
  if(x.size()==0) return false;
  for(size_t i = 0; i < x.size(); ++i) {
    if(R_ExternalPtrAddr(x[i]) == NULL) {
      return false;  
    }
  }
  return true;
}

setpointers <- function(x) {
  x@shlib$pointers <- pointers(x, refresh = TRUE)
  x
}

getpointers <- function(x) {
  x@shlib$pointers  
}