Relaxed load to skip invalid global attributes

The restrictive attributes dictionary prevents users adding inappropriate metadata to the attributes (e.g. standard_name) that should be stored elsewhere on the cube/coordinate. Unfortunately this means that files that do not follow the guidelines in appendix A in the CF spec may fail to load. Some data from NASA has a calendar attribute that is global. It should really be on the time coordinate variable, but it is not. As Iris currently stands this cannot be loaded as an exception is raised when the cf load mechanism attempt to add a calendar to the cube's attributes dictionary.

This PR relaxes the load from CF so that unsupported/invalid attribute keys are skipped, rather than raising an exception.