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