Describing ParameterGroup objects more explicitly

[chris-little]

See previous CovJSON issue covjson/specification#81

[chris-little]

The gist of the closed and archived issue is that there a few intrinsically different types of ParameterGroup:

vector: Children are components of a vector field
statistics: Children are different statistics of the same quantity
probdf: Children hold the values of the components of the probability density function
samples: Children hold the values from individual realisations of a sampling

There may be other useful groups.

(CoverageJSON already supports arbitrary grouping of parameters - the issue is standardising common useful use cases)