scifio: Add BaseMetadata

BaseMetadata is a base interface for MetadataStore and
MetadataRetrieve, giving the following hierarchy:

        BaseMetadata
           /    \
MetadataStore  MetadataRetrieve
           \    /
         IMetadata

The intention here is to have base type which can be used to
specialise containers for that type, which can contain instances
of both MetadataStore and MetadataRetrieve, principally
AggregateMeta.
Display reader name in the console during metadata pages auto-generation
Fix anchor formatting in metadata-summary page
Merge pull request #468 from rleigh-dundee/ibasemetadata

scifio: Add BaseMetadata
Add fix for generating metadata support with auto-generated OME-XML
Regenerate metadata pages
Merge pull request #474 from sbesson/rebased/develop/schema_anchors

Fix broken anchors in Sphinx metadata pages (rebased onto develop)