Separate Tsunami on Population IF from Flood Population IF

This PR will:
- addresses https://github.com/AIFDR/inasafe/issues/1238 (Default value - tsunami impact on people.)
- addresses part of https://github.com/AIFDR/inasafe/issues/502 (Name of Tsunami Impact Function is not valid). To address this fully I need to change other IF (Flood IF on building and on roads), but they don't really give different behavior between Flood and Tsunami. Any suggestion @Charlotte-Morgan  @cchristelis @ismailsunni ?