Rename getCenter as getMedian and getLength as getSpan

The following Envelope methods are GeoAPI extensions, not ISO 19107 methods:

• getCenter(int), named "center" by analogy with Rectangle2D.getCenterX/Y() in standard Java library.
• getLength(int).

They are misnamed, especially getLength(int) which suggests some kind of linear unit (e.g. the length of some polygon border). The proposal is to rename them as:

• getMedian(int)
• getSpan(int)

Note that "Span" is a name used in standard Java library for the same purpose than our intend: http://java.sun.com/javase/6/docs/api/javax/swing/text/BoxView.html#getSpan(int,%20int)

Their javadoc should be improved in the process with a clear definition of what those methods are expected to compute.