MeasureMap specification

Currently the specification consists of two JSONschemas:

• measure.schema.json: the fields that a single measure map entry may have
• map.schema.json: in its current form, it's simply an array of Measure objects, but it could be extended to include, for example, a version field.

The separation gives us flexibility in terms of various JSONschemas we might want to define for different types of measure maps. For example, the current map.schema.json is equivalent to JSON lines format, adding a version field could be separate schema, incompatible with JSONL.