implement subcollection for SpatialIndexFeatureCollection [GEOT-5310]

[ianturton]

Implements subcollection to GEOT-5310. This allows users to cut and paste from the documentation and actually have it work. Also speeds up spatial queries.

[aaime]

Looking good. I'd add a copyright header to SpatialIndexFeatureCollection, it's missing one completely.
According to the SVN logs it was contributed by @jodygarnett back in 2010:

> svn log http://svn.osgeo.org/geotools/trunk/modules/library/main/src/main/java/org/geotools/data/collection/SpatialIndexFeatureCollection.java
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r38089 | mbedward | 2011-09-27 04:21:51 +0200 (mar, 27 set 2011) | 1 line

Updating source taglet in class javadocs
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r37292 | mbedward | 2011-05-25 05:24:35 +0200 (mer, 25 mag 2011) | 1 line

updated source taglet in class javadocs
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r37093 | jive | 2011-05-05 12:33:45 +0200 (gio, 05 mag 2011) | 1 line

added faq in lack of for each loop support; added some helper constructors to the various explicit feature collection implementations
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r36527 | aaime | 2011-02-04 19:21:37 +0100 (ven, 04 feb 2011) | 1 line

[GEOT-3392] Allow SLD styles to drive data transformations before actual rendering
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r35587 | jive | 2010-05-25 16:14:50 +0200 (mar, 25 mag 2010) | 1 line

Working on a spatial index feature source
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r35568 | jive | 2010-05-24 16:50:39 +0200 (lun, 24 mag 2010) | 1 line

Initial SpatialIndexFeatureCollection implementation, need to make a feature source before this is useful to the renderer
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