Add a project .clang-format config file (#1053)

Scripts and workflows that run `clang-format` currently each add the
option `--style=google` to the invocation of `clang-format`. To follow
best practices, this PR adds a `.clang-format` configuration file at the
top level of the project to set the style, and removes the flag from
where it was added on the command line explicitly.

The use of a config file has the following benefits:

- It lets tools like IDEs discover the setting automatically.
- It reduces the chances that developers will forget to add the flag
when they run `clang-format` directly on the command line for quick
checks, or write new scripts that use `clang-format`.
- It communicates intentions more clearly to new contributors.

Removing the flag from the scripts is not strictly necessary (everything
will work the same if there is both a config file and the flag is passed
to `clang-format`). However, if a config file is used, it's better for
maintainability and consistency to also remove the flag from individual
invocations.

Author: mhucka

.clang-format

@@ -0,0 +1,3 @@
+# Run manually to reformat a file:
+# clang-format -i --style=file <file>
+BasedOnStyle: Google

scripts/format_all.sh

@@ -19,6 +19,6 @@ python3 -m yapf --style=google --in-place --recursive --parallel \
 echo -e "Done! \nDoing notebook formatting..."
 python3 ./scripts/format_ipynb.py
 echo -e "Done! \nDoing C++ formatting..."
-find tensorflow_quantum/ -iname *.h -o -iname *.cc | xargs clang-format -i -style=google
+find tensorflow_quantum/ -iname *.h -o -iname *.cc | xargs clang-format -i
 echo "Done!"
 exit 0;

scripts/format_check.sh

@@ -1,12 +1,12 @@
 #!/bin/bash
 # Copyright 2020 The TensorFlow Quantum Authors. All Rights Reserved.
-# 
+#
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
-# 
+#
 #     http://www.apache.org/licenses/LICENSE-2.0
-# 
+#
 # Unless required by applicable law or agreed to in writing, software
 # distributed under the License is distributed on an "AS IS" BASIS,
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
@@ -100,7 +100,7 @@ if [ $? -ne 0 ]; then
 fi
 
 echo "Checking C++ formatting...";
-formatting_outputs=$(find tensorflow_quantum/ -iname *.h -o -iname *.cc | xargs clang-format -style=google -output-replacements-xml);
+formatting_outputs=$(find tensorflow_quantum/ -iname *.h -o -iname *.cc | xargs clang-format -output-replacements-xml);
 CFORMATCHECK=0
 while read -r formatting_outputs; do
     if [ "$formatting_outputs" != "<?xml version='1.0'?>" ] && [ "$formatting_outputs" != "<replacements xml:space='preserve' incomplete_format='false'>" ] && [ "$formatting_outputs" != "</replacements>" ] && [ "$formatting_outputs" != "<replacement offset='4290' length='9'> </replacement>" ]; then