Don't hang if a user inputs "LAD" into run_ssm.py

[tg137]

Currently, if a user were to run the following:

python scripts/run_ssm.py -c config/ssm_default.json LAD

run_ssm.py would run apparently as normal until doing synthesizing the 2011 seed population (line 187 of utils.py https://github.com/nismod/microsimulation/blob/master/microsimulation/utils.py#L187)

The execution would then hang until the user forces a keyboard interrupt. 'LAD' should be treated as an invalid input value.