improved shell script reg. usability

perform_notebook_qa.sh

-cd notebooks
-py.test --nbval $1 
-nbqa isort $1
-nbqa black --line-length=79 $1
-nbqa flake8 $1
+#!/bin/bash
+
+# script takes a Jupyter Notebook as argument and calls
+# validation and static code analysis tools for this notebook
+
+# usage help
+usage() { 
+  echo "Usage: $0 NOTEBOOK_TO_CHECK.ipynb" 1>&2 
+}
+
+# call tools for given notebook
+perform_qa() {
+  py.test --nbval $1
+  nbqa isort $1
+  nbqa black --line-length=79 $1
+  nbqa flake8 $1
+}
+
+# check if command line argument is present
+if [ "$#" -ne 1 ]; then
+    echo "No notebook (.ipynb) was provided";
+    usage;
+    exit 1;
+fi
+
+perform_qa $1