Add nc_del_attr to Cdunif to allow attribute removal in netCDF files

[dnadeau4]

Allow to delete Global Attributes or Variable attributes when calling del(f.attribute) or delattr(f, attribute). Cdunifmodule "set_attribute" function is called with a null pointer "0x00" which is different than Py_None. Without this change deleting an attribute with a "del" or "delattr" call was crashing the program with a segmentation fault.

[doutriaux1]

@dnadeau4 can you please add a test case? Thanks!

import cdms2, MV
f = cdms2.open("test.nc","w")
s = MV2.ones((20,20))
s.id="test"
s.test_attribute = "some variable attribute"
f.test_attribute = "some file attribute"
f.write(s)
f.close()
f = cdms2.open("test.nc","r+")
del(f.some_attribute)
s=f["test"]
del(s.some_attribute)
f.close()
f = cdms2.open("test.nc")
assert(hasattr(f,"some_attribute") is False)
s=f["test"]
assert(hasattr(s,"some_attribute") is False)

[dnadeau4]

I added the test.

[durack1]

@dnadeau4 it would be a good idea to merge these changes into the existing branch issue1579_durack1_UpdateNetcdf4.3.3.1To4.4.0 - this branch includes the new netcdf 4.4.0 final release..

[doutriaux1]

@durack1 totally different issue, one issue per branch. The NetCDF4 needs testing first.

[durack1]

@doutriaux1 not sure I agree with you. In 4.4.0 Unidata/netcdf-c#128 is fixed, and these changes will likely impact the operation of Cdunif. This is documented in #1398

[dnadeau4]

@doutriaux1 How come you did not merge it yet? I did add the test and I tried it. Thanks for providing a template.

[doutriaux1]

my bad, didn't see that you had a test in!