several low level flw and raster methods

[DirkEilander]

new:

• flw.d8_from_dem to derive a flow direction raster from a DEM
• flw.reproject_hydrography_like to reproject flow direction raster data
• raster.flipud method to flip data along y-axis
• raster.area_grid to get the raster cell areas [m2]
• raster.density_grid to convert the values to [unit/m2]
• gis_utils.spread2d method wrapping its pyflwdir equivalent

deprecation warnings added to:

• flw.gaugemap is replaced by flw.gauge_map for a more conistent interace of flw.*map methods
• flw.basin_shape is redundant

[hboisgon]

Very nice new functions implementation from the new pyflwdir version! They're also already added in the docs and finally some units tests on the flw and raster methods :) Thanks @DirkEilander