MSatEnv: Handling of mathsat environment is now delegated to MSatEnv
MSatEnv: Handling of mathsat environment is now delegated to MSatEnv

pySMT classes should pass around (and store) references to MSatEnv.
In order to call the mathsat API we use the __call__ method thus
writing:

  env = MSatEnv()
  mathsat.function(env())

pysmt/pysmt,pysmt/pysmt,agriggio/pysmt,agriggio/pysmt