🐛 Bug Report

Nonsensical attributes are added to radar data when reading from nimrod file format

How To Reproduce

Steps to reproduce the behaviour:

1. Retrieve a nimrod file from the radar MASS or FRASIA archives (or I can provide a test file on request)
2. Read it in using iris.load_cube
3. In addition to useful attributes, attributes "recursive_filter_iterations" and "probability_period_of_event" have been added. These are not relevant except to IMPROVER and I suspect this may come from a fix made by @MoseleyS to the nimrod reader at some point.

Expected behaviour

Not having attributes added by iris that are not relevant to the data.

Environment

• OS & Version: RHEL7 (VDI)
• Iris Version: 3.1.0 (error is new since 3.0.1)

Additional details

The attributes I get from reading in a random hourly accumulation file are as follows:

{'nimrod_version': 2,
 'field_code': 214,
 'recursive_filter_iterations': 94,
 'probability_period_of_event': 0,
 'source': 'Radar composite\x00\x00\x00\x00\x00\x00\x00\x00\x00',
 'title': 'Unknown',
 'institution': 'Met Office'}

So there is also a formatting error with "source" that was also present in iris version 3.0.1, that needs to be fixed as well.