New ignore_properties keyword to cfdm.unique_constructs

[davidhassell]

It would be useful to allow cfdm.unique_constructs to ignore selected properties when assessing equality of input constructs. For example, two Field constructs may differ only by a timestamp within their "history" properties, and this may be of no concern. This would look like:

>>> import cfdm
>>> f = cfdm.example_field(0)
>>> f2 = f.copy()
>>> f.set_property('foo', 'bar')
>>> f2.set_property('foo', 'bar2')
>>> cfdm.unique_constructs([f, f2]) 
[<Field: specific_humidity(latitude(5), longitude(8)) 1>,
 <Field: specific_humidity(latitude(5), longitude(8)) 1>]
>>> cfdm.unique_constructs([f, f2], ignore_properties='foo')
[<Field: specific_humidity(latitude(5), longitude(8)) 1>]

[sadielbartholomew]

Nice idea! Reviewing the PRs now.