Check DEM vcrs when subtracting DEMs

[adehecq]

Just another thought as I'm going through the new doc.
To avoid frequent errors from users who difference two DEMs, then notice a large vertical bias and apply coregistration, one could maybe add a check during numpy operations on DEMs that the two VCRS are identical? This sounds easy, but maybe a bit annoying to implement, as one would need to overwrite the Raster's methods no?

Because the vcrs is not always defined, the way I see is:
if both vcrs are defined
if both vcrs are identical -> runs without message
if both vcrs are different -> raises an error with a message indicating how to change the vcrs
if not both vcrs are not defined -> raises a warning of missing information