Fixes #309: implement Environment::isMasterRank()
Fixes #308:
 - implement `evaluate()` for Ginac Expression 
 - the argument passed to evaluate provides a map of parameters (not
 necessarily all of them) to the 
 - the default values for the parameters/symbols are either 0 or the value of the
(command line) option having the same name as the parameter/symbol
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop