fixed unintended usage of operator f2 variable

- added member to eca requests: compare type
- eca functons now use new request member instead of cdo_operator_f2()
- added functions:
  * set_default_compare_type(&int)
  * set_compare_type_from_params(&int, params)A
- new functions set request member compare_type
- replaced oper_t manipulation with the new functions and use of the new
  member
- removed function addWithFrequency
- With these changes each function that uses the Eca routines needs to
  call the set_default_compare_type function, or rather is required to
  set the request member compare_type as the default is -1
- added assertion for request.compare_type != -1 in EcaIndices