[yt-dev] [yt_analysis/yt] Docs for Ghost Zone Requiring Covering Grids (issue #541)

[Sam Skillman]

New issue 541: Docs for Ghost Zone Requiring Covering Grids
https://bitbucket.org/yt_analysis/yt/issue/541/docs-for-ghost-zone-requiring-covering

Sam Skillman:

We should make it clear how to get a covering grid for fields that require ghost zones.  This is primarily a docs issue, but the 

By running the cookbook ``extract_fixed_resolution_data.py`` with DivV instead leads to this error:

```
#!python
skillman/test_data:python extract_fixed_resolution_data.py
Traceback (most recent call last):
  File "extract_fixed_resolution_data.py", line 19, in <module>
    num_ghost_zones=0)
  File "/home/skillman/local/src/yt-hg/yt/data_objects/data_containers.py", line 3735, in __init__
    self._refresh_data()
  File "/home/skillman/local/src/yt-hg/yt/data_objects/data_containers.py", line 3752, in _refresh_data
    AMR3DData._refresh_data(self)
  File "/home/skillman/local/src/yt-hg/yt/data_objects/data_containers.py", line 320, in _refresh_data
    self.get_data()
  File "/home/skillman/local/src/yt-hg/yt/data_objects/data_containers.py", line 3770, in get_data
    self._generate_field(field)
  File "/home/skillman/local/src/yt-hg/yt/data_objects/data_containers.py", line 3801, in _generate_field
    self.pf.field_info[field].check_available(self)
  File "/home/skillman/local/src/yt-hg/yt/data_objects/field_info_container.py", line 364, in check_available
    validator(data)
  File "/home/skillman/local/src/yt-hg/yt/data_objects/field_info_container.py", line 494, in __call__
    raise NeedsGridType(self.ghost_zones,self.fields)
yt.data_objects.field_info_container.NeedsGridType: (1, ['x-velocity', 'y-velocity', 'z-velocity'])


```

We should catch this and report back to the user to require the correct number of ghost zones

Responsible: samskillman
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