add clang-format configuration file
In the discussion on the opal-developers mailing list about the code formatting tool there was a - not ver strong - preference to use clang-format, mainly due to it simpler configuration.
clang-format is able to support our coding-style. Nevertheless in some cases reformatting is a bit ugly. In the worst-case reformatting can easily be disabled for some lines of code by enclosing them in
// clang-format off add
// clang-format on.