Added lint pre-commit hook

Added a quick pre-commit hook that will apply the black formatter with
the same params used in the GitHub action.

bin/pre-commit

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+if which black > /dev/null 2>&1; then
+    black --line-length 92 \
+        --extend-exclude 'flepimop/gempyor_pkg/src/gempyor/steps_rk4.py' \
+        --check --verbose .
+else
+    echo "'black' is not available so python files will not be checked."
+fi