Do not overwrite an existing process.options in TimeMemorySummary customise

The customise function in TimeMemorySummary.py was overwriting a
process.options if one was already defined. When this customise was
used in conjunction with the customise which turned on threading,
this one turned off the threading.
Merge pull request #3485 from Dr15Jones/doNotClobberOptionsPSetInTimeMemorySummary

Do not overwrite an existing process.options in TimeMemorySummary customise