Search for fields by prefix (and suggest prefixes).

The best practise is still to specify the exact field to be visualised.

However, the default for "field" is now None, not 'BtEn', so there's a small possibility of a breaking change for those relying on this, who also specified a numerical radius. As now field = None causes a search based on field_prefix = 'Bt', the search could pick up BtE500 before BtEn for example.

This is an edge case to let a new convenience helper work, not a major API change, so I have only bumped a minor version number.