Clean up compiler pragmas

UG uses pragmas in a few places, most of which appear to be obsolete (to me). Try to find out what they do (or did) to see if they are still relevant, and get rid of as many of them as possible.

I could not find these pragmas. Is this still an issue?

I removed most #pragma directives by now (in 009484fd or by removing files). There is only one #pragma unused left which can be replaced with DUNE_UNUSED_PARAMETER; I'll submit a merge request for it.

Sadly C++17's [[maybe_unused]] triggers a warning with some pre-C++17 compilers.

mentioned in merge request !99 (merged)

closed via commit d2624ccc

closed via commit 78ef341c

closed via merge request !99 (merged)

mentioned in commit d2624ccc