Need validations during data preprocessing stage

[dyuliu]

When running Orion on certain datasets, we may encounter the following errors.

The start_time or stop_time of one signal is a strange negative value.

This error results from the invalid timestamp values existing on the raw data file. We have to do a format validation here I think when we load signals from certain locations.

This bug has a very harmful impact: if the start_time is a negative value and let's say the interval we used to aggregate data is 6 hours. We will then create huge amount of "unnecessary" intervals starting from the wrong start_time.