Use a new helper, nilmdb.utils.time.float_to_time_string().
This will help if we ever want to change representation (like using
uint64 microseconds since epoch, which saves us from having to
waste bits on the floating-point exponent)
This is mostly a matter of taste, but it matches more closely with the
old way that prep did it, and it's more consistent. It should roughly
match the available precision of floats and doubles.
