Maximising Inheritance Principle: Optional suffices

[Lestropie]

Extension of #2.

Relates to proposal in bids-standard/bids-2-devel#75.

When seeking the highest filesystem level & minimal number of entities at which to place any given metadata field value, there persists the constraint that the metadata file must have the same suffix to any data file to which it is deemed applicable. This makes it impossible to take metadata fields that are common across images of different contrasts and define them just once. This could however be quite useful, were version 2 of the specification to permit it.

• Add command-line toggle to specify whether metadata files can be defined without a suffix.
• Modulate algorithm responsible for exploiting Inheritance Principle to minimise duplication of metadata, by toggling whether or not metadata files must have the same suffix as all applicable data files (or equivalently, for any given data file, only read from metadata files that have the same suffix, in addition to the other criteria of the Inheritance Principle).