bulkmap context: default configuration

[sbesson]

See https://forum.image.sc/t/are-attributes-in-an-images-table-searchable/27999/5

A configuration file is mandatory in the current version of the library (0.4.0) to use the bulkmap context and transform bulk annotations into map annotations. It should be possible to support a default configuration/behavior. The most natural expectation is that every column of the table should be turned into a key/value pair per image with a default namespace.

In terms of specification, this probably means reviewing the bulkmap options and defining default values as well as default behaviors if a field is not defined. This review is something that will be needed in order to fix #32. A further step would be the formalization of a proper schema.