Refactor omc session classes (#250)

* merge OMCSessionHelper into OMCSessionZMQ

* define execute as depreciated

* merge (parts of) OMCSessionBase into OMCSessionZMQ

* class OMCSessionBase only contains ask() and derived functions to call OM
* the moved functions were using variables of class OMCSessionZMQ
* this change splits the class definitions into
  (1) code to connect to OM => OMCSessionBase
  (2) code to execute OM / connect to running OM => OMCSessionZMQ

Author: syntron