Add pre-commit for automatically updating date stamps (#660)

# Add pre-commit for automatically updating date stamps

Closes #658.

Works for me but worth please check this works for you too.

.pre-commit

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+# An example pre-commit file for nextSIM-DG development
+
+for FILE in $(git diff --cached --name-only | grep -iE '\.(cpp|cc|h|hpp)$'); do
+  # Apply clang-format for linting
+  clang-format -i ${FILE} --verbose
+  # Update date stamp to today's date
+  LINE=$(grep '@date' ${FILE})
+  let SPACES=$(echo "${LINE}" | tr -cd ' \t' | wc -c)-4
+  NEWLINE=$(printf "%s%$((SPACES))s%s\n" " @date" " " "$(date '+%d %b %Y')")
+  sed -i~ "s/${LINE}/${NEWLINE}/" ${FILE}
+done

README.md

@@ -23,7 +23,7 @@ For neXtSIM_DG we use clang-format and the [Webkit style](https://webkit.org/cod
   -  Names and values of physical constants reside in src/include/constants.hpp
   -  Runtime modifiable model parameters are set in src/options.cpp and can be set using option files.
 
-A [dedicated clang format file](https://github.com/nextsimdg/nextsimdg/blob/issue9_clang_format/src/.clang-format) has been designed for the code. You may run it loccaly and manually with the command ```clang-format -i $yourfile``` or have a plugin with your favorite code editor or implement a git pre-commit hook locally by putting this pre-commit file in your .git/hooks/. This clang formatting will also be run each time a pull request is done as part of the continuous integration. 
+A [dedicated clang format file](https://github.com/nextsimhub/nextsimdg/blob/main/.clang-format) has been designed for the code. You may run it locally and manually with the command ```clang-format -i $yourfile``` or have a plugin with your favorite code editor or implement a git pre-commit hook locally by putting this pre-commit file in your .git/hooks/. An example pre-commit file can be found at [.pre-commit](https://github.com/nextsimhub/nextsimdg/blob/658_pre-commit-date/.pre-commit). This clang formatting will also be run each time a pull request is done as part of the continuous integration.
 
 
 ## Commenting conventions for a nice automatic documentation
@@ -47,6 +47,7 @@ Example :
 */
 
 ```
+Note that the example [pre-commit](https://github.com/nextsimhub/nextsimdg/blob/658_pre-commit-date/.pre-commit) file mentioned above will also automatically update this if moved to `.git/hooks/pre-commit`.
 
 We ask every coders to follow the following commenting conventions for comments providing the automatic documentation:
   -  documentation blocks must appear before the component it describes, with the same indentation (classes and functions),