Fix HEAppE prepare job operator

Hi, thanks for your great work on the LEXIS/HEAppE Airflow components! While trying out the new HEAppEPrepareJobOperatorFullContext, I encountered two issues (that were also easily resolved):

• The HEAppEPrepareJobOperatorFullContext was passing a WalltimeLimit argument to the HEAppEPrepareJobParametersAPISpec, but this did not exist in the HEAppEPrepareJobParametersAPISpec class. I replaced it with the WaitingLimit argument, as that seemed to be used in other places and the WalltimeLimit argument can be set per individual task.
• The HEAppEPrepareJobParametersAPISpec always replaced the MaxCores argument with the static number 128. This meant you could never allocate more nodes than this static number. I simply removed this override and that seemed to do the trick.

Please let me know if these fixes are in line with how the classes are meant to be used!