Import loss-calculator packages correctly

The losscalculator packages such as fragility_model, ground_motion & losslib are imported in the damage_calculator.py incorrectly. they need to be imported as below:

from losscalculator.fragility_model import FragilityModel
from losscalculator.ground_motion import GMField
from losscalculator.losslib import get_full_gm_field