Refactor the nd2 source to use the nd2 library.

This should be more consistent than the nd2reader library.

The only downside of this is that the nd2 module is Python 3.7 or later.

setup.py

@@ -1,5 +1,6 @@
 import itertools
 import os
+import sys
 
 from setuptools import setup
 
@@ -21,14 +22,17 @@
     'gdal': ['large-image-source-gdal'],
     'mapnik': ['large-image-source-mapnik'],
     'multi': ['large-image-source-multi'],
-    'nd2': ['large-image-source-nd2'],
     'ometiff': ['large-image-source-ometiff'],
     'openjpeg': ['large-image-source-openjpeg'],
     'openslide': ['large-image-source-openslide'],
     'pil': ['large-image-source-pil'],
     'test': ['large-image-source-test'],
     'tiff': ['large-image-source-tiff'],
 }
+if sys.version_info >= (3, 7):
+    sources.update({
+        'nd2': ['large-image-source-nd2'],
+    })
 extraReqs.update(sources)
 extraReqs['sources'] = list(set(itertools.chain.from_iterable(sources.values())))
 extraReqs['all'] = list(set(itertools.chain.from_iterable(extraReqs.values())))