SciPy dependency

[dopplershift]

Description

Currently we unconditionally import scipy in two places:

cartopy/lib/cartopy/img_transform.py

Lines 13 to 18 in d12c86c

	try:
	import pykdtree.kdtree
	_is_pykdtree = True
	except ImportError:
	import scipy.spatial
	_is_pykdtree = False

(when no pykdtree) and here:

cartopy/lib/cartopy/vector_transform.py

Lines 12 to 14 in d59d275

	import numpy as np
	from scipy.interpolate import griddata

That would contradict where we list scipy as an "optional" dependency. We should either update the docs in INSTALL or handle those imports differently.

[jackieleng]

Ran into this issue today. Followed the installation guide and thought I installed all deps. Tried running the animate_surface.ipynb notebook, but failed, because I have no pykdtree nor scipy installed

[greglucas]

We also list matplotlib as an optional dependency, which seems a bit odd :) So, that should probably be updated as well.