readshapefile returns a multipolygon as several polygons

まだ誰も着手していません。

I wanted to draw a choropleth map using basemap. My plan was to read the polygons from the shapefile vis `readshapefile` and then further process them using `shapely`, `descartes`, etc.

The problem that I encountered was that `readshapefile` returned multiple polygons for each `MultiPolygon`. This causes confusion, as for the enclosed polygon we will have more than one record.

I think the fix will be to return a `MultiPolygon` instead of multiple polygons.

Here is an example:

``` python
from collections import OrderedDict
import fiona
from mpl_toolkits.basemap import Basemap

rec1 = dict()
rec2 = dict()

rec1['id'] = 0
rec1['type'] = 'Feature'
rec1['properties'] = OrderedDict([(u'NAME', 'outer'),(u'VALUE', 100)])
rec1['geometry'] = dict()
rec1['geometry']['type'] = 'MultiPolygon'
rec1['geometry']['coordinates'] = [[[(-50,40),(50,40),
(50,-40),(-50,-40),
(-50,40)
]],
[[(-45,35),(45,35),
(45,-35),(-45,-35),
(-45,35)
]]]

rec2['id'] = 1
rec2['type'] = 'Feature'
rec2['properties'] = OrderedDict([(u'NAME', 'inner'),(u'VALUE', 1)])
rec2['geometry'] = dict()
rec2['geometry']['type'] = 'Polygon'
rec2['geometry']['coordinates'] = [[(-45,35),(45,35),
(45,-35),(-45,-35),
(-45,35)
]]
driver = 'ESRI Shapefile'
crs = {'no_defs': True, 'ellps': 'WGS84', 'datum': 'WGS84', 'proj': 'longlat'}
schema = {'geometry': 'Polygon',
'properties': {'NAME': 'str',
'VALUE': 'float:15.2'}}

with fiona.open(
'/tmp/foo.shp',
'w',
driver=driver,
crs=crs,
schema=schema) as c:
c.write(rec1)
c.write(rec2)

m = Basemap()
m.readshapefile('/tmp/foo','foo')
for info,region in zip(m.foo_info, m.foo):
print info
print region
```

for which the output is:

```
{'RINGNUM': 1, 'NAME': 'outer', 'VALUE': 100.0, 'SHAPENUM': 1}
[(-50.0, 40.0), (50.0, 40.0), (50.0, -40.0), (-50.0, -40.0), (-50.0, 40.0)]
{'RINGNUM': 2, 'NAME': 'outer', 'VALUE': 100.0, 'SHAPENUM': 1}
[(-45.0, 35.0), (-45.0, -35.0), (45.0, -35.0), (45.0, 35.0), (-45.0, 35.0)]
{'RINGNUM': 1, 'NAME': 'inner', 'VALUE': 1.0, 'SHAPENUM': 2}
[(-45.0, 35.0), (45.0, 35.0), (45.0, -35.0), (-45.0, -35.0), (-45.0, 35.0)]
```

エントリーポイント Basemap.readshapefile から始め、提供された Fiona の例を再現し、返されたレコードをソースジオメトリと比較します。MultiPolygon のケースが 1 つのソース feature のジオメトリとメタデータを保持し、別の Polygon のケースを壊さなければ完了です。

索引モデルが issue の本文から書いたものです。