297 - Rework raster layer to read on load not on get data.
Fix failing test (due to minimum needs update).
Added nodata handling standardization to only use nan's. Update loaded data to have nodata values set to nan's.
Updated test behaviour to match new nan for nodata behaviour.
Merge branch 'develop' of github.com:AIFDR/inasafe into develop
Merge pull request #1210 from cchristelis/develop

Refactor Raster data load and nodata behaviour - Issue #297