Allow reference areas to be filtered by name

[lkitching]

Allow observations to be filtered by (possibly partial) matches on the reference area label e.g.

 observations(dimensions: {reference_area: {label: "E01004803"}}) { ... }

regular expressions for labels could also be supported e.g.

observations(dimensions: {reference_area: {label_regex: "E0.*"}}) { ... }

along with matching by URI (the current behaviour)

observations(dimensions: {reference_area: {uri: "http://statistics.data.gov.uk/id/statistical-geography/E01004803"}}) { ... }