Changed scipy and numpy deps to normal pip install

For some reason, Scipy and Numpy dependencies were marked as ones where you had to install them system-wide. I'm not sure why this was: presumably something to do with the history of Scipy and Numpy development.

Nowadays, as far as I know, there's no reason not to install Scipy and Numpy in a virtual environment, so I've changed the standard dependencies to normal pip installations.

src/python/pimlico/core/dependencies/python.py

@@ -310,12 +310,8 @@ def get_installed_version(self, local_config):
 # Some commonly used dependencies #
 ###################################
 
-numpy_dependency = PythonPackageSystemwideInstall("numpy", "Numpy",
-                                                  pip_package="numpy", yum_package="numpy", apt_package="python-numpy",
-                                                  url="http://www.numpy.org/")
-scipy_dependency = PythonPackageSystemwideInstall("scipy", "Scipy",
-                                                  pip_package="scipy", yum_package="scipy", apt_package="python-scipy",
-                                                  url="https://www.scipy.org/scipylib/")
+numpy_dependency = PythonPackageOnPip("numpy", "Numpy")
+scipy_dependency = PythonPackageOnPip("scipy", "Scipy")
 theano_dependency = PythonPackageOnPip("theano", pip_package="Theano")
 tensorflow_dependency = PythonPackageOnPip("tensorflow")
 # We usually need h5py for reading/storing models