Introduce transforms which operate solely on metadata

Introduces MetadataTransform and UntrainableMetadataTransform, which
provide a project(file,file) interface for defining operations that only
affect metadata, and do not touch matrices.

Modify a number of transforms to inherit from these interfaces instead of
UntrainableTransform or UntrainableMetaTransform
Merge pull request #184 from biometrics/metadata_transform

Introduce transforms which operate solely on metadata