check presence of attribute and variables

[Alexander-Barth]

The following code should be an error:

using NCDatasets
using Test
sz = (4,5)
filename = tempname()
#filename = "/tmp/mytest.nc"


ds = NCDataset(filename,"c")
ds.attrib["comment"] = "foo"
"comment" in ds.attrib

and not return true, as in

"foo" in Dict("foo" => "bar")

Presence of attributes and variables should be checked with haskey.