Reformat the code-base with clang-format 6.0 (#957)

Description: Reformat the code-base with clang-format 6.0.

Details:
Formatting changes

angle brackets without spaces (allowed to remove the chevron-fixer)
reorder includes (within a block) in alphabetic order (this is the default in the new version, I kept it as I think it makes sense)
git-hook updated: chevron fixer removed, version check updated
Actions to be performed by the developers

get clang-format-6.0.0
update the git hook with the one from this branch
format all files which where changed in your open branch
merge with this branch, i.e. master once this PR is in
Note that you might get some merge conflicts if you clang-format everything in your branch and then try to merge with this branch (as I had to manually add some includes which were implicitly included prior to this PR, but were not available after alphabetic ordering).
On daint and kesch clang-format-6.0.0 is available with module load /users/vogtha/modules/tools/clang-format/6.0.0

Author: havogt

Diff for: .clang-format

@@ -44,7 +44,7 @@ TabWidth:        8
 UseTab:          Never
 BreakBeforeBraces: Attach
 SpacesInParentheses: false
-SpacesInAngles:  true
+SpacesInAngles:  false
 SpaceInEmptyParentheses: false
 SpacesInCStyleCastParentheses: false
 SpacesInContainerLiterals: true