Make sure all raster data has `Point` `Sampling`

[mauro3]

Quite confusingly raster data can have point or interval sampling. That also impacts on what the coordinates refer to, https://github.com/eth-vaw-glaciology/GlacioTools.jl#raster-data-as-point-or-area-data----and----as-center-coordinates.

I propose that we exclusively use Point sampling. I have a made an assertion function which checks that that is the case:

GlacioTools.jl/src/Rasters_helpers.jl

Line 125 in e0f9b53

function assert_Point(ra::Union{AbstractRaster, AbstractRasterStack})

which can be used like so:

GlacioTools.jl/src/Antarctica.jl

Line 127 in e0f9b53

return assert_Point(RasterStack(gas..., metadata=Rasters.metadata(nc), dims=(x, y))), nc

TODO:

• bedmap2 is interval-like
• most geotiffs are, me thinks