Build dynamic library by default

Using a statically linked dune-uggrid together with the Python bindings leads to hard-to-diagnose crashes and other bugs. This patch therefore enables shared libraries, and adds a lengthy explanation of why this is necessary.

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• 5cb7a295 - Build dynamic library by default

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mentioned in merge request core/dune-common!1422

The same for dune-common: core/dune-common!1422

Does this fix the bug revealed by core/dune-grid!737 ?

I didn't explicitly test this, but I am nevertheless quite certain that it does.

Same as in dune-common, at least it should be set as an option and not as a hard variable.