Added more detailed docs.

Added
=====

* More, numpy formatted, docs for landlab.field subpackage.
* New exception GroupError and FieldError.

Changes
=======

* Raise GroupError or FieldError instead of KeyError when trying to get
  a field that does not exist in the collection. For backward
  compatibility, both of these exceptions inherit from KeyError.
Fixed unit tests for new exceptions, name changes.