storage_access() should be deprecated at some point...

E.g. print 1 instead of [1] for scalar vectors, [1 2] instead of [[1 2]]
for matrices with only one row...

If you did rely on `common/logging.hh` to include `common/timedlogging.hh`
your code may not compile anymore.

is_hashable<T>::value is true iff std::hash<T>(T t) works or gcc is too
old

I see no harm in it atm.

-#include <dune/stuff/common/logging.hh>
+#include <dune/stuff/common/timedlogging.hh>
is encouraged, but not mandatory yet

This does not yet test whether the file has actually been created or
if it contains the correct stuff.

This commit should get rid of some warnings and small inconsistencies.
The LogStream class now derives from StorageProvider which holds the
buffer before the buffer is given to the ostream base class. FileLogStream
and EmptyLogStream now merely create the corresponding stream buffer
and forward it to LogStream. Thus overriding rdbuf does not seem necessary
any more.