automate aggregating over multiple runs

[stueller]

In the 2nd simulation study, power curves were not smooth. To check whether it was a bad seed, we reran the study a couple of times. We get similar jaggadness but for different conditions. In practice, we would like to be able to aggregate across replications to check whether B was large enough (i.e., does jaggedness smooth out after stacking).

[stueller]

This can probably be done with an update to the $start() method:

• add an option like 'append' set to FALSE by default (if set to TRUE and there are no files already, ignore)
• update it to always run in a temporary folder or write to temporary files
• if append=TRUE and the current directory has output append the csv and update the other output files (mse, txt)
• add annotation to .complete noting that the condition was appended to (this will require changes to checking the .complete file)