Change type of volume method to typedef

This is a rebased version of !305 (closed) --- I seem unable to rebase that one in situ.

added 1 commit

• e06f16d8 - Make Geometry export the return type of the 'volume' method

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mentioned in merge request !305 (closed)

I changed the Geometry interface class to determine the return type of the implementation's volume method automatically. That way, downstream grid implementations do not have to be adapted.

merged

mentioned in commit 94015683