Update INSTALL to be consistent

Author: znicholls

INSTALL

@@ -75,7 +75,7 @@ Hence the commands change to::
     python requirements/gen_conda_requirements.py --groups test docs > conda_requirements.txt
     conda create -n my_iris_env --file conda_requirements.txt
     conda activate my_iris_env  # or whatever other name you gave it
-    python setup.py develop
+    pip install -e .
 
 The tests can then be run with