[PNM][util] add poreExtendedRadius to dgf-file

What this MR does / why does DuMux need it:

Fixes #1254 (closed)

Notes for the reviewer

Before there was only one radius written into .dgf-files which was PoreInscribedRadius, but inside the script saved under the name pore.radius. For pore.radius the extended pore radius of the network was assigned.

Now, we distinguish between the inscribed and the extended radius. The PoreInscribedRadius/pore.inscribedRadius and PoreExtendedRadius/pore.extendedRadius is now obtained from the inscribed and extended radius of the network. If the inscribed radius is not part of the parameters of the network, the extended radius of the network will be assigned to pore.inscribedRadius as it was done before.

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