Draft: Allow setting policies by variables

Summary

With !1269 (merged) we introduced a policy system into the cmake buildsystem. The problem is that you need to explicitly set a policy in the cmake files in order to activate or deactivate a new feature. Sometimes it might be useful to set a policy globally for all modules without changing the cmake files, e.g. for testing a new policy. With the MR one can set a policy from commandline, by -DDUNE_POLICY_DPxxxx=[NEW|OLD] directly. However, the manually set policies in the cmake files with dune_set_policy(DPxxxx [NEW|OLD]) has precedence over the commandline parameters.