initial commit of pre-commit configuration

To use the pre-commit hooks install `pre-commit` with the following
instructions:

  https://pre-commit.com/#install

then run ONCE in your CP2K clone:

    pre-commit install

After that - before every commit - all the scripts configured in
`.pre-commit-config.yaml` will run on the files added in that
commit.

If any file changed (for example because they were modified by the
`prettify` script), the commit will be interrupted for you to
investigate and adding the prettyfied file to the commit if
necessary.

.pre-commit-config.yaml

@@ -0,0 +1,21 @@
+# See https://pre-commit.com for more information
+# See https://pre-commit.com/hooks.html for more hooks
+repos:
+-   repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v2.0.0
+    hooks:
+    -   id: check-yaml
+    -   id: check-added-large-files
+-   repo: local
+    hooks:
+    -   id: prettify
+        name: Run prettify
+        entry: ./tools/prettify/prettify.py
+        language: script
+        files: '.*\.(F|f90)$'
+        exclude: >-
+          (?x)^(
+            tools/.*|
+            src/base/base_uses.f90|
+            src/common/util.F|
+          )$