Changing density term to count.

Hi @timlinux and @Charlotte-Morgan, this PR:
- addresses https://github.com/AIFDR/inasafe/issues/1233.
- needs translation update (There is one failing test that needs translation update from transifex)
- needs inasafe_data update. The PR for the data is: https://github.com/AIFDR/inasafe_data/pull/4